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NAME
    Mail::Box - manage a mailbox, a folder with messages

INHERITANCE
     Mail::Box
       is a Mail::Reporter

     Mail::Box is extended by
       Mail::Box::Dir
       Mail::Box::File
       Mail::Box::Net

SYNOPSIS
     use Mail::Box::Manager;
     my $mgr    = Mail::Box::Manager->new;
     my $folder = $mgr->open(folder => $ENV{MAIL}, ...);
     print $folder->name;

     # Get the first message.
     print $folder->message(0);

     # Delete the third message
     $folder->message(3)->delete;

     # Get the number of messages in scalar context.
     my $emails = $folder->messages;

     # Iterate over the messages.
     foreach ($folder->messages) {...} # all messages
     foreach (@$folder) {...}          # all messages

     $folder->addMessage(Mail::Box::Message->new(...));

    Tied-interface:

     tie my(@inbox), 'Mail::Box::Tie::ARRAY', $inbox;

     # Four times the same:
     $inbox[3]->print;                 # tied
     $folder->[3]->print;              # overloaded folder
     $folder->message(3)->print;       # usual
     print $folder->[3];               # overloaded message

     tie my(%inbox), 'Mail::Box::Tie::HASH', $inbox;

     # Twice times the same
     $inbox{$msgid}->print;            # tied
     $folder->messageId($msgid)->print;# usual

DESCRIPTION
    A Mail::Box::Manager creates "Mail::Box" objects. But you already knew, because you started with
    the Mail::Box-Overview manual page. That page is obligatory reading, sorry!

    "Mail::Box" is the base class for accessing various types of mailboxes (folders) in a uniform
    manner. The various folder types vary on how they store their messages, but when some effort
    those differences could be hidden behind a general API. For example, some folders store many
    messages in one single file, where other store each message in a separate file within the same
    directory.

    No object in your program will be of type "Mail::Box": it is only used as base class for the
    real folder types. "Mail::Box" is extended by

    Extends "DESCRIPTION" in Mail::Reporter.

OVERLOADED
    overload: ""
        (stringification) The folder objects stringify to their name. This simplifies especially
        print statements and sorting a lot.

        example: use overloaded folder as string

         # Three lines with overloading: resp. cmp, @{}, and ""
         foreach my $folder (sort @folders)
         {   my $msgcount = @$folder;
             print "$folder contains $msgcount messages\n";
         }

    overload: @{}
        When the folder is used as if it is a reference to an array, it will show the messages, like
        messages() and message() would do.

        example: use overloaded folder as array

         my $msg = $folder->[3];
         my $msg = $folder->message(3);          # same

         foreach my $msg (@$folder) ...
         foreach my $msg ($folder->messages) ... # same

    overload: cmp
        (string comparison) folders are compared based on their name. The sort rules are those of
        the build-in "cmp".

METHODS
    Extends "METHODS" in Mail::Reporter.

  Constructors
    Extends "Constructors" in Mail::Reporter.

    Mail::Box->new(%options)
        Open a new folder. A list of labeled %options for the mailbox can be supplied. Some options
        pertain to Mail::Box, and others are added by sub-classes.

        To control delay-loading of messages, as well the headers as the bodies, a set of *_type
        options are available. "extract" determines whether we want delay-loading.

         -Option           --Defined in     --Default
          access                              'r'
          body_delayed_type                   Mail::Message::Body::Delayed
          body_type                           <folder specific>
          coerce_options                      []
          create                              <false>
          extract                             10240
          field_type                          undef
          fix_headers                         <false>
          folder                              $ENV{MAIL}
          folderdir                           undef
          head_delayed_type                   Mail::Message::Head::Delayed
          head_type                           Mail::Message::Head::Complete
          keep_dups                           <false>
          lock_file                           undef
          lock_timeout                        1 hour
          lock_type                           Mail::Box::Locker::DotLock
          lock_wait                           10 seconds
          locker                              undef
          log                Mail::Reporter   'WARNINGS'
          manager                             undef
          message_type                        <folder-class>::Message
          multipart_type                      Mail::Message::Body::Multipart
          remove_when_empty                   <true>
          save_on_exit                        <true>
          trace              Mail::Reporter   'WARNINGS'
          trusted                             <depends on folder location>

        access => MODE
          Access-rights to the folder. Folders are opened for read-only (which means
          write-protected) by default! MODE can be

          'r': read-only (default)
          'a': append
          'rw': read-write
          'd': delete

          These MODE has no relation to the modes actually used to open the folder files within this
          module. For instance, if you specify "rw", and open the folder, only read permission on
          the folder-file is required.

          Be warned: writing a MBOX folder may create a new file to replace the old folder. The
          permissions and owner of the file may get changed by this.

        body_delayed_type => CLASS
          The bodies which are delayed: which will be read from file when it is needed, but not
          before.

        body_type => CLASS|CODE
          When messages are read from a folder-file, the headers will be stored in a "head_type"
          object. For the body, however, there is a range of choices about type, which are all
          described in Mail::Message::Body.

          Specify a CODE-reference which produces the body-type to be created, or a CLASS of the
          body which is used when the body is not a multipart or nested. In case of a code
          reference, the header structure is passed as first argument to the routine.

          Do *not* return a delayed body-type (like "::Delayed"), because that is determined by the
          "extract" option while the folder is opened. Even delayed message will require some real
          body type when they get parsed eventually. Multiparts and nested messages are also outside
          your control.

          For instance:

           $mgr->open('InBox', body_type => \&which_body);

           sub which_body($) {
               my $head = shift;
               my $size = $head->guessBodySize || 0;
               my $type = $size > 100000 ? 'File' : 'Lines';
               "Mail::Message::Body::$type";
           }

          The default depends on the mail-folder type, although the general default is
          Mail::Message::Body::Lines. Please check the applicable manual pages.

        coerce_options => ARRAY
          Keep configuration information for messages which are coerced into the specified folder
          type, starting with a different folder type (or even no folder at all). Messages which are
          coerced are always fully read, so this kind of information does not need to be kept here.

        create => BOOLEAN
          Automatically create the folder when it does not exist yet. This will only work when
          access is granted for writing or appending to the folder.

          Be careful: you may create a different folder type than you expect unless you explicitly
          specify Mail::Box::Manager::open(type).

        extract => INTEGER | CODE | METHOD | 'LAZY'|'ALWAYS'
          Defines when to parse (process) the content of the message. When the header of a message
          is read, you may want to postpone the reading of the body: header information is more
          often needed than the body data, so why parse it always together? The cost of delaying is
          not too high, and with some luck you may never need parsing the body.

          If you supply an INTEGER to this option, bodies of those messages with a total size less
          than that number will be extracted from the folder only when necessary. Messages where the
          size (in the "Content-Length" field) is not included in the header, like often the case
          for multiparts and nested messages, will not be extracted by default.

          If you supply a CODE reference, that subroutine is called every time that the extraction
          mechanism wants to determine whether to parse the body or not. The subroutine is called
          with the following arguments:

           CODE->(FOLDER, HEAD)

          where FOLDER is a reference to the folder we are reading. HEAD refers to the
          Mail::Message::Head::Complete head of the message at hand. The routine must return a
          "true" value (extract now) or a "false" value (be lazy, do not parse yet). Think about
          using the Mail::Message::Head::guessBodySize() and Mail::Message::guessTimestamp() on the
          header to determine your choice.

          The third possibility is to specify the NAME of a method. In that case, for each message
          is called:

           FOLDER->NAME(HEAD)

          Where each component has the same meaning as described above.

          The fourth way to use this option involves constants: with "LAZY" all messages will be
          delayed. With "ALWAYS" you enforce unconditional parsing, no delaying will take place. The
          latter is usuful when you are sure you always need all the messages in the folder.

           $folder->new(extract => 'LAZY');  # Very lazy
           $folder->new(extract => 10000);   # Less than 10kB

           # same, but implemented yourself
           $folder->new(extract => &large);
           sub large($) {
              my ($f, $head) = @_;
              my $size = $head->guessBodySize;
              defined $size ? $size < 10000 : 1
           };

           # method call by name, useful for Mail::Box
           # extensions. The example selects all messages
           # sent by you to be loaded without delay.
           # Other messages will be delayed.
           $folder->new(extract => 'sent_by_me');
           sub Mail::Box::send_by_me($) {
               my ($self, $header) = @_;
               $header->get('from') =~ m/\bmy\@example.com\b/i;
           }

        field_type => CLASS
          The type of the fields to be used in a header. Must extend Mail::Message::Field.

        fix_headers => BOOLEAN
          Broken MIME headers usually stop the parser: all lines not parsed are added to the body of
          the message. With this flag set, the erroneous line is added to the previous header field
          and parsing is continued. See Mail::Box::Parser::Perl::new(fix_header_errors).

        folder => FOLDERNAME
          Which folder to open (for reading or writing). When used for reading (the "access" option
          set to "r" or "a") the mailbox should already exist and must be readable. The file or
          directory of the mailbox need not exist if it is opened for reading and writing ("rw").
          Write-permission is checked when opening an existing mailbox.

          The folder name can be preceded by a "=", to indicate that it is named relative to the
          directory specified in new(folderdir). Otherwise, it is taken as relative or absolute
          path.

        folderdir => DIRECTORY
          Where are folders to be found by default? A folder-name may be preceded by a equals-sign
          ("=", a "mutt" convension) to explicitly state that the folder is located below the
          default directory. For example: in case "folderdir => '/tmp'" and "folder => '=abc'", the
          name of the folder-file is '/tmp/abc'. Each folder type has already some default set.

        head_delayed_type => CLASS
          The headers which are delayed: which will be read from file when it is needed, but not
          before.

        head_type => CLASS
          The type of header which contains all header information. Must extend
          Mail::Message::Head::Complete.

        keep_dups => BOOLEAN
          Indicates whether or not duplicate messages within the folder should be retained. A
          message is considered to be a duplicate if its message-id is the same as a previously
          parsed message within the same folder. If this option is false (the default) such messages
          are automatically deleted, because it is considered useless to store the same message
          twice.

        lock_file => FILENAME
          The name of the file which is used to lock. This must be specified when locking is to be
          used.

        lock_timeout => SECONDS
          When the lock file is older than the specified number of SECONDS, it is considered a
          mistake. The original lock is released, and accepted for this folder.

        lock_type => CLASS|STRING|ARRAY
          The type of the locker object. This may be the full name of a CLASS which extends
          Mail::Box::Locker, or one of the known locker types "DotLock", "Flock", "FcntlLock",
          "Mutt", "NFS", "POSIX", or "NONE". If an ARRAY is specified, then a Multi locker is built
          which uses the specified list.

        lock_wait => SECONDS
          SECONDS to wait before failing on opening this folder.

        locker => OBJECT
          An OBJECT which extends Mail::Box::Locker, and will handle folder locking replacing the
          default lock behavior.

        log => LEVEL
        manager => MANAGER
          A reference to the object which manages this folder -- typically an Mail::Box::Manager
          instance.

        message_type => CLASS
          What kind of message objects are stored in this type of folder. The default is constructed
          from the folder class followed by "::Message". For instance, the message type for
          "Mail::Box::POP3" is "Mail::Box::POP3::Message"

        multipart_type => CLASS
          The default type of objects which are to be created for multipart message bodies.

        remove_when_empty => BOOLEAN
          Determines whether to remove the folder file or directory automatically when the write
          would result in a folder without messages nor sub-folders.

        save_on_exit => BOOLEAN
          Sets the policy for saving the folder when it is closed. A folder can be closed manually
          (see close()) or in a number of implicit ways, including on the moment the program is
          terminated.

        trace => LEVEL
        trusted => BOOLEAN
          Flags whether to trust the data in the folder or not. Folders which reside in your
          "folderdir" will be trusted by default (even when the names if not specified staring with
          "="). Folders which are outside the folderdir or read from STDIN
          (Mail::Message::Construct::read()) are not trused by default, and require some extra
          checking.

          If you do not check encodings of received messages, you may print binary data to the
          screen, which is a security risk.

  The folder
    $obj->addMessage($message, %options)
        Add a message to the folder. A message is usually a Mail::Box::Message object or a sub-class
        thereof. The message shall not be in an other folder, when you use this method. In case it
        is, use Mail::Box::Manager::moveMessage() or Mail::Box::Manager::copyMessage() via the
        manager.

        Messages with id's which already exist in this folder are not added.

        BE WARNED that message labels may get lost when a message is moved from one folder type to
        an other. An attempt is made to translate labels, but there are many differences in
        interpretation by applications.

         -Option--Default
          share   <not used>

        share => BOOLEAN
          Try to share the physical resource of the current message with the indicated message. It
          is sometimes possible to share messages between different folder types. When the sharing
          is not possible, than this option is simply ignored.

          Sharing the resource is quite dangerous, and only available for a limited number of folder
          types, at the moment only some Mail::Box::Dir folders; these file-based messages can be
          hardlinked (on platforms that support it). The link may get broken when one message is
          modified in one of the folders.... but maybe not, depending on the folder types involved.

        example:

         $folder->addMessage($msg);
         $folder->addMessages($msg1, $msg2, ...);

    $obj->addMessages(@messages)
        Adds a set of message objects to the open folder at once. For some folder types this may be
        faster than adding them one at a time.

        example:

         $folder->addMessages($msg1, $msg2, ...);

    Mail::Box->appendMessages(%options)
        Append one or more messages to an unopened folder. Usually, this method is called by the
        Mail::Box::Manager::appendMessage(), in which case the correctness of the folder type is
        checked.

        For some folder types it is required to open the folder before it can be used for appending.
        This can be fast, but this can also be very slow (depends on the implementation). All
        %options passed will also be used to open the folder, if needed.

         -Option  --Default
          folder    <required>
          message   undef
          messages  undef
          share     <false>

        folder => FOLDERNAME
          The name of the folder to which the messages are to be appended. The folder implementation
          will avoid opening the folder when possible, because this is resource consuming.

        message => MESSAGE
        messages => ARRAY-OF-MESSAGES
          One reference to a MESSAGE or a reference to an ARRAY of MESSAGEs, which may be of any
          type. The messages will be first coerced into the correct message type to fit in the
          folder, and then will be added to it.

        share => BOOLEAN
          Try to share physical storage of the message. Only available for a limited number of
          folder types, otherwise no-op.

        example:

         my $message = Mail::Message->new(...);
         Mail::Box::Mbox->appendMessages
          ( folder    => '=xyz'
          , message   => $message
          , folderdir => $ENV{FOLDERS}
          );

        better:

         my Mail::Box::Manager $mgr;
         $mgr->appendMessages($message, folder => '=xyz');

    $obj->close(%options)
        Close the folder, which usually implies writing the changes. This will return "false" when
        writing is required but fails. Please do check this result.

        WARNING: When moving messages from one folder to another, be sure to write the destination
        folder before writing and closing the source folder. Otherwise you may lose data if the
        system crashes or if there are software problems.

         -Option      --Default
          force         <false>
          save_deleted  false
          write         MODIFIED

        force => BOOLEAN
          Override the new(access) setting which was specified when the folder was opened. This
          option only has an effect if its value is TRUE. NOTE: Writing to the folder may not be
          permitted by the operating system, in which case even "force" will not help.

        save_deleted => BOOLEAN
          Do also write messages which where flagged to be deleted to their folder. The flag for
          deletion is conserved (when possible), which means that a re-open of the folder may remove
          the messages for real. See write(save_deleted).

        write => 'ALWAYS'|'NEVER'|'MODIFIED'
          Specifies whether the folder should be written. As could be expected, "ALWAYS" means
          always (even if there are no changes), "NEVER" means that changes to the folder will be
          lost, and "MODIFIED" only saves the folder if there are any changes.

        example:

         my $f = $mgr->open('spam', access => 'rw')
             or die "Cannot open spam: $!\n";

         $f->message(0)->delete
             if $f->messages;

         $f->close
             or die "Couldn't write $f: $!\n";

    $obj->copyTo($folder, %options)
        Copy the folder's messages to a new folder. The new folder may be of a different type.

         -Option       --Default
          delete_copied  <false>
          select         'ACTIVE'
          share          <not used>
          subfolders     <folder type dependent>

        delete_copied => BOOLEAN
          Flag the messages from the source folder to be deleted, just after it was copied. The
          deletion will only take effect when the originating folder is closed.

        select => 'ACTIVE'|'DELETED'|'ALL'|LABEL|!LABEL|FILTER
          Which messages are to be copied. See the description of messages() about how this works.

        share => BOOLEAN
          Try to share the message between the folders. Some Mail::Box::Dir folder types do support
          it by creating a hardlink (on UNIX/Linux).

        subfolders => BOOLEAN|'FLATTEN'|'RECURSE'
          How to handle sub-folders. When false (0 or "undef"), sub-folders are simply ignored. With
          "FLATTEN", messages from sub-folders are included in the main copy. "RECURSE" recursively
          copies the sub-folders as well. By default, when the destination folder supports
          sub-folders "RECURSE" is used, otherwise "FLATTEN". A value of true will select the
          default.

        example:

         my $mgr  = Mail::Box::Manager->new;
         my $imap = $mgr->open(type => 'imap', host => ...);
         my $mh   = $mgr->open(type => 'mh', folder => '/tmp/mh',
             create => 1, access => 'w');

         $imap->copyTo($mh, delete_copied => 1);
         $mh->close; $imap->close;

    $obj->delete(%options)
        Remove the specified folder file or folder directory (depending on the type of folder) from
        disk. Of course, THIS IS DANGEROUS: you "may" lose data. Returns a "true" value on success.

        WARNING: When moving messages from one folder to another, be sure to write the destination
        folder before deleting the source folder. Otherwise you may lose data if the system crashes
        or if there are software problems.

         -Option   --Default
          recursive  1

        recursive => BOOLEAN

        example: removing an open folder

         my $folder = Mail::Box::Mbox->new(folder => 'InBox', access => 'rw');
         ... some other code ...
         $folder->delete;

        example: removing an closed folder

         my $folder = Mail::Box::Mbox->new(folder => 'INBOX', access => 'd');
         $folder->delete;

    $obj->folderdir( [$directory] )
        Get or set the $directory which is used to store mail-folders by default.

        example:

         print $folder->folderdir;
         $folder->folderdir("$ENV{HOME}/nsmail");

    $obj->name()
        Returns the name of the folder. What the name represents depends on the actual type of
        mailbox used.

        example:

         print $folder->name;
         print "$folder";       # overloaded stringification

    $obj->organization()
        Returns how the folder is organized: as one "FILE" with many messages, a "DIRECTORY" with
        one message per file, or by a "REMOTE" server.

    $obj->size()
        Returns the size of the folder in bytes, not counting in the deleted messages. The error in
        the presented result may be as large as 10%, because the in-memory representation of
        messages is not always the same as the size when they are written.

    $obj->type()
        Returns a name for the type of mail box. This can be "mbox", "mh", "maildir", or "pop3".

    $obj->update(%options)
        Read new messages from the folder, which where received after opening it. This is quite
        dangerous and shouldn't be possible: folders which are open are locked. However, some
        applications do not use locks or the wrong kind of locks. This method reads the changes (not
        always failsafe) and incorporates them in the open folder administration.

        The %options are extra values which are passed to the updateMessages() method which is doing
        the actual work here.

    $obj->url()
        Represent the folder as a URL (Universal Resource Locator) string. You may pass such a URL
        as folder name to Mail::Box::Manager::open().

        example:

         print $folder->url;
         # may result in
         #   mbox:/var/mail/markov   or
         #   pop3://user:password AT pop.com:101

  Folder flags
    $obj->access()
        Returns the access mode of the folder, as set by new(access)

    $obj->isModified()
        Checks if the folder, as stored in memory, is modified. A true value is returned when any of
        the messages is to be deleted, has changed, or messages were added after the folder was read
        from file.

        WARNING: this flag is not related to an external change to the folder structure on disk.
        Have a look at update() for that.

    $obj->modified( [BOOLEAN] )
        Sets whether the folder is modified or not.

    $obj->writable()
        Checks whether the current folder is writable.

        example:

         $folder->addMessage($msg) if $folder->writable;

  The messages
    $obj->current( [$number|$message|$message_id] )
        Some mail-readers keep the *current* message, which represents the last used message. This
        method returns [after setting] the current message. You may specify a $number, to specify
        that that message number is to be selected as current, or a $message/$message_id (as long as
        you are sure that the header is already loaded, otherwise they are not recognized).

        example:

         $folder->current(0);
         $folder->current($message);

    $obj->find($message_id)
        Like messageId(), this method searches for a message with the $message_id, returning the
        corresponding message object. However, "find" will cause unparsed message in the folder to
        be parsed until the message-id is found. The folder will be scanned back to front.

    $obj->findFirstLabeled( $label, [BOOLEAN, [$msgs]] )
        Find the first message which has this $label with the correct setting. The BOOLEAN indicates
        whether any true value or any false value is to be found in the ARRAY of $msgs. By default,
        a true value is searched for. When a message does not have the requested label, it is taken
        as false.

        example: looking for a labeled message

         my $current = $folder->findFirstLabeled('current');

         my $first   = $folder->findFirstLabeled(seen => 0);

         my $last    = $folder->findFirstLabeled(seen => 0,
                         [ reverse $self->messages('ACTIVE') ] )

    $obj->message( $index, [$message] )
        Get or set a message with on a certain index. Messages which are flagged for deletion are
        counted. Negative indexes start at the end of the folder.

        example:

         my $msg = $folder->message(3);
         $folder->message(3)->delete;   # status changes to `deleted'
         $folder->message(3, $msg);
         print $folder->message(-1);    # last message.

    $obj->messageId( $message_id, [$message] )
        With one argument, returns the message in the folder with the specified $message_id. If a
        reference to a message object is passed as the optional second argument, the message is
        first stored in the folder, replacing any existing message whose message ID is $message_id.
        (The message ID of $message need not match $message_id.)

        !!WARNING!!: when the message headers are delay-parsed, the message might be in the folder
        but not yet parsed into memory. In this case, use find() instead of "messageId()" if you
        really need a thorough search. This is especially the case for directory organized folders
        without special indexi, like Mail::Box::MH.

        The $message_id may still be in angles, which will be stripped. In that case blanks (which
        origin from header line folding) are removed too. Other info around the angles will be
        removed too.

        example:

         my $msg = $folder->messageId('<complex-message.id>');
         $folder->messageId("<complex-message\n.id>", $msg);
         my $msg = $folder->messageId('complex-message.id');
         my $msg = $folder->messageId('garbage <complex-message.id> trash');

    $obj->messageIds()
        Returns a list of *all* message-ids in the folder, including those of messages which are to
        be deleted.

        For some folder-types (like MH), this method may cause all message-files to be read. See
        their respective manual pages.

        example:

         foreach my $id ($folder->messageIds) {
            $folder->messageId($id)->print;
         }

    $obj->messages( <'ALL'|$range|'ACTIVE'|'DELETED'|$label| !$label|$filter> )
        Returns multiple messages from the folder. The default is "ALL" which will return (as
        expected maybe) all the messages in the folder. The "ACTIVE" flag will return the messages
        not flagged for deletion. This is the opposite of "DELETED", which returns all messages from
        the folder which will be deleted when the folder is closed.

        You may also specify a $range: two numbers specifying begin and end index in the array of
        messages. Negative indexes count from the end of the folder. When an index is out-of-range,
        the returned list will be shorter without complaints.

        Everything else than the predefined names is seen as labels. The messages which have that
        label set will be returned. When the sequence starts with an exclamation mark (!), the
        search result is reversed.

        For more complex searches, you can specify a $filter, which is simply a code reference. The
        message is passed as only argument.

        example:

         foreach my $message ($folder->messages) {...}
         foreach my $message (@$folder) {...}

         # twice the same
         my @messages   = $folder->messages;
         my @messages   = $folder->messages('ALL');

         # Selection based on a range (begin, end)
         my $subset     = $folder->messages(10,-8);

         # twice the same:
         my @not_deleted= grep {not $_->isDeleted}
                             $folder->messages;
         my @not_deleted= $folder->messages('ACTIVE');

         # scalar context the number of messages
         my $nr_of_msgs = $folder->messages;

         # third message, via overloading
         $folder->[2];

         # Selection based on labels
         $mgr->moveMessages($spam, $inbox->message('spam'));
         $mgr->moveMessages($archive, $inbox->message('seen'));

    $obj->nrMessages(%options)
        Simply calls messages() in scalar context to return a count instead of the messages itself.
        Some people seem to understand this better. Note that nrMessages() will default to returning
        a count of "ALL" messages in the folder, including both "ACTIVE" and "DELETED".

        The %options are passed to (and explained in) messages().

    $obj->scanForMessages($message, $message_ids, $timespan, $window)
        You start with a $message, and are looking for a set of messages which are related to it.
        For instance, messages which appear in the 'In-Reply-To' and 'Reference' header fields of
        that message. These messages are known by their $message_ids and you want to find them in
        the folder.

        When all message-ids are known, then looking-up messages is simple: they are found in a
        plain hash using messageId(). But Mail::Box is lazy where it can, so many messages may not
        have been read from file yet, and that's the preferred situation, because that saves time
        and memory.

        It is not smart to search for the messages from front to back in the folder: the chances are
        much higher that related message reside closely to each other. Therefore, this method starts
        scanning the folder from the specified $message, back to the front of the folder.

        The $timespan can be used to terminate the search based on the time enclosed in the message.
        When the constant string "EVER" is used as $timespan, then the search is not limited by
        that. When an integer is specified, it will be used as absolute time in time-ticks as
        provided by your platform dependent "time" function. In other cases, it is passed to
        timespan2seconds() to determine the threshold as time relative to the message's time.

        The $window is used to limit the search in number of messages to be scanned as integer or
        constant string "ALL".

        Returned are the message-ids which were not found during the scan. Be warned that a
        message-id could already be known and therefore not found: check that first.

        example: scanning through a folder for a message

         my $refs   = $msg->get('References') or return;
         my @msgids = $ref =~ m/\<([^>]+\>/g;
         my @failed = $folder->scanForMessages($msg, \@msgids, '3 days', 50);

  Sub-folders
    $obj->listSubFolders(%options)
    Mail::Box->listSubFolders(%options)
        List the names of all sub-folders to this folder, not recursively decending. Use these names
        as argument to openSubFolder(), to get access to that folder.

        For MBOX folders, sub-folders are simulated.

         -Option    --Default
          check       <false>
          folder      <from calling object>
          folderdir   <from folder>
          skip_empty  <false>

        check => BOOLEAN
          Should all returned foldernames be checked to be sure that they are of the right type?
          Each sub-folder may need to be opened to check this, with a folder type dependent penalty
          (in some cases very expensive).

        folder => FOLDERNAME
          The folder whose sub-folders should be listed.

        folderdir => DIRECTORY
        skip_empty => BOOL
          Shall empty folders (folders which currently do not contain any messages) be included?
          Empty folders are not useful to open, but may be useful to save to.

        example:

         my $folder = $mgr->open('=in/new');
         my @subs = $folder->listSubFolders;

         my @subs = Mail::Box::Mbox->listSubFolders(folder => '=in/new');
         my @subs = Mail::Box::Mbox->listSubFolders; # toplevel folders.

    $obj->nameOfSubFolder( $subname, [$parentname] )
    Mail::Box->nameOfSubFolder( $subname, [$parentname] )
        Returns the constructed name of the folder with NAME, which is a sub-folder of this current
        one. You have either to call this method as instance method, or specify a $parentname.

        example: how to get the name of a subfolder

         my $sub = Mail::Box::Mbox->nameOfSubfolder('xyz', 'abc');
         print $sub;                        # abc/xyz

         my $f = Mail::Box::Mbox->new(folder => 'abc');
         print $f->nameOfSubfolder('xyz');  # abc/xyz

         my $sub = Mail::Box::Mbox->nameOfSubfolder('xyz', undef);
         print $sub;                        # xyz

    $obj->openRelatedFolder(%options)
        Open a folder (usually a sub-folder) with the same options as this one. If there is a folder
        manager in use, it will be informed about this new folder. %options overrule the options
        which where used for the folder this method is called upon.

    $obj->openSubFolder($subname, %options)
        Open (or create, if it does not exist yet) a new subfolder in an existing folder.

        example:

         my $folder = Mail::Box::Mbox->new(folder => '=Inbox');
         my $sub    = $folder->openSubFolder('read');

    $obj->topFolderWithMessages()
    Mail::Box->topFolderWithMessages()
        Some folder types can have messages in the top-level folder, some other can't.

  Internals
    $obj->coerce($message, %options)
        Coerce the $message to be of the correct type to be placed in the folder. You can specify
        Mail::Internet and MIME::Entity objects here: they will be translated into Mail::Message
        messages first.

    $obj->create($foldername, %options)
    Mail::Box->create($foldername, %options)
        Create a folder. If the folder already exists, it will be left unchanged. The folder is
        created, but not opened! If you want to open a file which may need to be created, then use
        Mail::Box::Manager::open() with the create flag, or Mail::Box::new(create).

         -Option   --Default
          folderdir  undef

        folderdir => DIRECTORY
          When the foldername is preceded by a "=", the "folderdir" directory will be searched for
          the named folder.

    $obj->determineBodyType($message, $head)
        Determine which kind of body will be created for this message when reading the folder
        initially.

    Mail::Box->foundIn( [$foldername], %options )
        Determine if the specified folder is of the type handled by the folder class. This method is
        extended by each folder sub-type.

        The $foldername specifies the name of the folder, as is specified by the application. You
        need to specified the "folder" option when you skip this first argument.

        %options is a list of extra information for the request. Read the documentation for each
        type of folder for type specific options, but each folder class will at least support the
        "folderdir" option:

         -Option   --Default
          folderdir  undef

        folderdir => DIRECTORY
          The location where the folders of this class are stored by default. If the user specifies
          a name starting with a "=", that indicates that the folder is to be found in this default
          DIRECTORY.

        example:

         Mail::Box::Mbox->foundIn('=markov',
             folderdir => "$ENV{HOME}/Mail");
         Mail::Box::MH->foundIn(folder => '=markov');

    $obj->lineSeparator( [<STRING|'CR'|'LF'|'CRLF'>] )
        Returns the character or characters used to separate lines in the folder file, optionally
        after setting it to STRING, or one of the constants. The first line of the folder sets the
        default.

        UNIX uses a LF character, Mac a CR, and Windows both a CR and a LF. Each separator will be
        represented by a "\n" within your program. However, when processing platform foreign
        folders, complications appear. Think about the "Size" field in the header.

        When the separator is changed, the whole folder me be rewritten. Although, that may not be
        required.

    $obj->locker()
        Returns the locking object.

    $obj->read(%options)
        Read messages from the folder into memory. The %options are folder specific. Do not call
        "read()" yourself: it will be called for you when you open the folder via the manager or
        instantiate a folder object directly.

        NOTE: if you are copying messages from one folder to another, use addMessages() instead of
        "read()".

        example:

         my $mgr = Mail::Box::Manager->new;
         my $folder = $mgr->open('InBox');             # implies read
         my $folder = Mail::Box::Mbox->new(folder => 'Inbox'); # same

    $obj->readMessages(%options)
        Called by read() to actually read the messages from one specific folder type. The read()
        organizes the general activities.

        The %options are "trusted", "head_type", "field_type", "message_type", "body_delayed_type",
        and "head_delayed_type" as defined by the folder at hand. The defaults are the constructor
        defaults (see new()).

    $obj->storeMessage($message)
        Store the message in the folder without the checks as performed by addMessage().

    $obj->toBeThreaded($messages)
        The specified message is ready to be removed from a thread. This will be passed on to the
        mail-manager, which keeps an overview on which thread-detection objects are floating around.

    $obj->toBeUnthreaded($messages)
        The specified message is ready to be included in a thread. This will be passed on to the
        mail-manager, which keeps an overview on which thread-detection objects are floating around.

    $obj->updateMessages(%options)
        Called by update() to read messages which arrived in the folder after it was opened.
        Sometimes, external applications dump messages in a folder without locking (or using a
        different lock than your application does).

        Although this is quite a dangerous, it only fails when a folder is updated (reordered or
        message removed) at exactly the same time as new messages arrive. These collisions are
        sparse.

        The options are the same as for readMessages().

    $obj->write(%options)
        Write the data to disk. The folder (a "true" value) is returned if successful. Deleted
        messages are transformed into destroyed messages: their memory is freed.

        WARNING: When moving messages from one folder to another, be sure to write (or close()) the
        destination folder before writing (or closing) the source folder: otherwise you may lose
        data if the system crashes or if there are software problems.

        To write a folder to a different file, you must first create a new folder, then move all the
        messages, and then write or close() that new folder.

         -Option      --Default
          force         <false>
          save_deleted  <false>

        force => BOOLEAN
          Override write-protection with new(access) while opening the folder (whenever possible, it
          may still be blocked by the operating system).

        save_deleted => BOOLEAN
          Do also write messages which where flagged to be deleted to their folder. The flag for
          deletion is conserved (when possible), which means that a re-open of the folder may remove
          the messages for real. See close(save_deleted).

    $obj->writeMessages(%options)
        Called by write() to actually write the messages from one specific folder type. The "write"
        organizes the general activities. All options to write() are passed to "writeMessages" as
        well. Besides, a few extra are added by "write" itself.

         -Option  --Default
          messages  <required>

        messages => ARRAY
          The messages to be written, which is a sub-set of all messages in the current folder.

  Other methods
    $obj->timespan2seconds($time)
    Mail::Box->timespan2seconds($time)
        $time is a string, which starts with a float, and then one of the words 'hour', 'hours',
        'day', 'days', 'week', or 'weeks'. For instance: '1 hour' or '4 weeks'.

  Error handling
    Extends "Error handling" in Mail::Reporter.

    $obj->AUTOLOAD()
        Inherited, see "Error handling" in Mail::Reporter

    $obj->addReport($object)
        Inherited, see "Error handling" in Mail::Reporter

    $obj->defaultTrace( [$level]|[$loglevel, $tracelevel]|[$level, $callback] )
    Mail::Box->defaultTrace( [$level]|[$loglevel, $tracelevel]|[$level, $callback] )
        Inherited, see "Error handling" in Mail::Reporter

    $obj->errors()
        Inherited, see "Error handling" in Mail::Reporter

    $obj->log( [$level, [$strings]] )
    Mail::Box->log( [$level, [$strings]] )
        Inherited, see "Error handling" in Mail::Reporter

    $obj->logPriority($level)
    Mail::Box->logPriority($level)
        Inherited, see "Error handling" in Mail::Reporter

    $obj->logSettings()
        Inherited, see "Error handling" in Mail::Reporter

    $obj->notImplemented()
        Inherited, see "Error handling" in Mail::Reporter

    $obj->report( [$level] )
        Inherited, see "Error handling" in Mail::Reporter

    $obj->reportAll( [$level] )
        Inherited, see "Error handling" in Mail::Reporter

    $obj->trace( [$level] )
        Inherited, see "Error handling" in Mail::Reporter

    $obj->warnings()
        Inherited, see "Error handling" in Mail::Reporter

  Cleanup
    Extends "Cleanup" in Mail::Reporter.

    $obj->DESTROY()
        This method is called by Perl when an folder-object is no longer accessible by the rest of
        the program.

DETAILS
  Different kinds of folders
    In general, there are three classes of folders: those who group messages per file, those who
    group messages in a directory, and those do not provide direct access to the message data. These
    folder types are each based on a different base class.

    *   File based folders Mail::Box::File

        File based folders maintain a folder (a set of messages) in one single file. The advantage
        is that your folder has only one single file to access, which speeds-up things when all
        messages must be accessed at once.

        One of the main disadvantages over directory based folders is that you have to construct
        some means to keep all message apart. For instance MBOX adds a message separator line
        between the messages in the file, and this line can cause confusion with the message's
        contents.

        Where access to all messages at once is faster in file based folders, access to a single
        message is (much) slower, because the whole folder must be read. However, in directory based
        folders you have to figure-out which message you need, which may be a hassle as well.

        Examples of file based folders are MBOX, DBX, and NetScape.

    *   Directory based folders Mail::Box::Dir

        In stead of collecting multiple messages in one file, you can also put each message in a
        separate file and collect those files in a directory to represent a folder.

        The main disadvantages of these folders are the enormous amount of tiny files you usually
        get in your file-system. It is extremely slow to search through your whole folder, because
        many files have to be opened to do so.

        The best feature of this organization is that each message is kept exactly as it was
        received, and can be processed with external scripts as well: you do not need any mail user
        agent (MUA).

        Examples of directory organized folders are MH, Maildir, EMH and XMH.

    *   Network (external) folders Mail::Box::Net

        Where both types described before provide direct access to the message data, maintain these
        folder types the message data for you: you have to request for messages or parts of them.
        These folders do not have a filename, file-system privileges and system locking to worry
        about, but typically require a hostname, folder and message IDs, and authorization.

        Examples of these folder types are the popular POP and IMAP, and database oriented message
        storage.

  Available folder types
    *   Mail::Box::Dbx (read only)

        Dbx files are created by Outlook Express. Using the external (optional) Mail::Transport::Dbx
        module, you can read these folders, even when you are running MailBox on a UNIX/Linux
        platform.

        Writing and deleting messages is not supported by the library, and therefore not by MailBox.
        Read access is enough to do folder conversions, for instance.

    *   Mail::Box::IMAP4 (partially)

        The IMAP protocol is very complex. Some parts are implemented to create (sub-optimal but
        usable) IMAP clients. Besides, there are also some parts for IMAP servers present. The most
        important lacking feature is support for encrypted connections.

    *   Mail::Box::Maildir

        Maildir folders have a directory for each folder. A folder directory contains "tmp", "new",
        and "cur" sub-directories, each containing messages with a different purpose. Files with new
        messages are created in "tmp", then moved to "new" (ready to be accepted). Later, they are
        moved to the "cur" directory (accepted). Each message is one file with a name starting with
        timestamp. The name also contains flags about the status of the message.

        Maildir folders can not be used on Windows by reason of file-name limitations on that
        platform.

    *   Mail::Box::Mbox

        A folder type in which all related messages are stored in one file. This is a very common
        folder type for UNIX.

    *   Mail::Box::MH

        This folder creates a directory for each folder, and a message is one file inside that
        directory. The message files are numbered sequentially on order of arrival. A special
        ".mh_sequences" file maintains flags about the messages.

    *   Mail::Box::POP3 (read/delete only)

        POP3 is a protocol which can be used to retrieve messages from a remote system. After the
        connection to a POP server is made, the messages can be looked at and removed as if they are
        on the local system.

    *   Mail::Box::Netzwert

        The Netzwert folder type is optimized for mailbox handling on a cluster of systems with a
        shared NFS storage. The code is not released under GPL (yet)

    Other folder types are on the (long) wishlist to get implemented. Please, help implementing more
    of them.

  Folder class implementation
    The class structure of folders is very close to that of messages. For instance, a
    Mail::Box::File::Message relates to a Mail::Box::File folder. The folder types are:

                        Mail::Box::Netzwert
     Mail::Box::Mbox   | Mail::Box::Maildir Mail::Box::POP3
     |  Mail::Box::Dbx | | Mail::Box::MH    |  Mail::Box::IMAP4
     |  |               | | |                 |  |
     |  |               | | |                 |  |
     Mail::Box::File   Mail::Box::Dir       Mail::Box::Net
           |                  |                   |
           `--------------.   |   .---------------'
                          |   |   |
                          Mail::Box
                              |
                              |
                        Mail::Reporter (general base class)

    By far most folder features are implemented in Mail::Box, so available to all folder types.
    Sometimes, features which appear in only some of the folder types are simulated for folders that
    miss them, like sub-folder support for MBOX.

DIAGNOSTICS
    Warning: Changes not written to read-only folder $self.
        You have opened the folder read-only --which is the default set by new(access)--, made
        modifications, and now want to close it. Set close(force) if you want to overrule the access
        mode, or close the folder with close(write) set to "NEVER".

    Error: Copying failed for one message.
        For some reason, for instance disc full, removed by external process, or read-protection, it
        is impossible to copy one of the messages. Copying will proceed for the other messages.

    Error: Destination folder $name is not writable.
        The folder where the messages are copied to is not opened with write access (see
        new(access)). This has no relation with write permission to the folder which is controlled
        by your operating system.

    Warning: Different messages with id $msgid
        The message id is discovered more than once within the same folder, but the content of the
        message seems to be different. This should not be possible: each message must be unique.

    Error: Folder $name is opened read-only
        You can not write to this folder unless you have opened the folder to write or append with
        new(access), or the "force" option is set true.

    Error: Folder $name not deleted: not writable.
        The folder must be opened with write access via new(access), otherwise removing it will be
        refused. So, you may have write-access according to the operating system, but that will not
        automatically mean that this "delete" method permits you to. The reverse remark is valid as
        well.

    Error: Invalid timespan '$timespan' specified.
        The string does not follow the strict rules of the time span syntax which is permitted as
        parameter.

    Warning: Message-id '$msgid' does not contain a domain.
        According to the RFCs, message-ids need to contain a unique random part, then an "@", and
        then a domain name. This is made to avoid the creation of two messages with the same id. The
        warning emerges when the "@" is missing from the string.

    Error: No folder name specified.
        You did not specify the name of a folder to be opened. Use the new(folder) option or set the
        "MAIL" environment variable.

    Error: Package $package does not implement $method.
        Fatal error: the specific package (or one of its superclasses) does not implement this
        method where it should. This message means that some other related classes do implement this
        method however the class at hand does not. Probably you should investigate this and probably
        inform the author of the package.

    Error: Unable to create subfolder $name of $folder.
        The copy includes the subfolders, but for some reason it was not possible to copy one of
        these. Copying will proceed for all other sub-folders.

    Error: Writing folder $name failed
        For some reason (you probably got more error messages about this problem) it is impossible
        to write the folder, although you should because there were changes made.

SEE ALSO
    This module is part of Mail-Box distribution version 3.009, built on August 18, 2020. Website:
    http://perl.overmeer.net/CPAN/

LICENSE
    Copyrights 2001-2020 by [Mark Overmeer]. For other contributors see ChangeLog.

    This program is free software; you can redistribute it and/or modify it under the same terms as
    Perl itself. See http://dev.perl.org/licenses/

Mail::Box(3pm)
NAME INHERITANCE SYNOPSIS DESCRIPTION OVERLOADED METHODS
Constructors The folder Folder flags The messages Sub-folders Internals Other methods Error handling Cleanup
DETAILS
Different kinds of folders Available folder types Folder class implementation
DIAGNOSTICS SEE ALSO LICENSE

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