maildir(5) - man - phpMan

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MAILDIR(5) Double Precision, Inc. MAILDIR(5)

NAME
maildir - E-mail directory

SYNOPSIS
$HOME/Maildir

DESCRIPTION
A “Maildir” is a structured directory that holds E-mail messages. Maildirs were first
implemented by the Qmail mail server. Qmail's maildirs were a simple data structure, nothing
more than a single collection of E-mail messages. The Courier mail server builds upon Qmail's
maildirs to provide extended functionality, such as folders and quotas. This document
describes the Courier mail server's extended maildirs, without explicitly identifying The
Courier mail server-specific extensions. See maildir(5) in Qmail's documentation for the
original definition of maildirs.

Traditionally, E-mail folders were saved as plain text files, called “mboxes”. Mboxes have
known limitations. Only one application can use an mbox at the same time. Locking is required
in order to allow simultaneous concurrent access by different applications. Locking is often
problematic, and not very reliable in network-based filesystem requirements. Some
network-based filesystems don't offer any reliable locking mechanism at all. Furthermore,
even bulletproof locking won't prevent occasional mbox corruption. A process can be killed or
terminated in the middle of updating an mbox. This will likely result in corruption, and a
loss of most messages in the mbox.

Maildirs allow multiple concurrent access by different applications. Maildirs do not require
locking. Multiple applications can update a maildir at the same time, without stepping on
each other's feet.

Maildir contents
A “maildir” is a directory that's created by maildirmake(1)[1]. Naturally, maildirs should
not have any group or world permissions, unless you want other people to read your mail. A
maildir contains three subdirectories: tmp, new, and cur. These three subdirectories comprise
the primary folder, where new mail is delivered by the system.

Folders are additional subdirectories in the maildir whose names begin with a period: such as
.Drafts or .Sent. Each folder itself contains the same three subdirectories, tmp, new, and
cur, and an additional zero-length file named maildirfolder, whose purpose is to inform any
mail delivery agent that it's really delivering to a folder, and that the mail delivery agent
should look in the parent directory for any maildir-related information.

Folders are not physically nested. A folder subdirectory, such as .Sent does not itself
contain any subfolders. The main maildir contains a single, flat list of subfolders. These
folders are logically nested, and periods serve to separate folder hierarchies. For example,
.Sent.2002 is considered to be a subfolder called “2002” which is a subfolder of “Sent”.

Folder name encoding
Folder names can contain any Unicode character, except for control characters. US-ASCII
characters, U+0x0020 - U+0x007F, except for the period, forward-slash, and ampersand
characters (U+0x002E, U+0x002F, and U+0x0026) represent themselves. The ampersand is
represent by the two character sequence “&-”. The period, forward slash, and non US-ASCII
Unicode characters are represented using the UTF-7 character set, and encoded with a
modified form of base64-encoding.

The “&” character starts the modified base64-encoded sequence; the sequence is terminated
by the “-” character. The sequence of 16-bit Unicode characters is written in big-endian
order, and encoded using the base64-encoding method described in section 5.2 of RFC
1521[2], with the following modifications:

• The “=” padding character is omitted. When decoding, an incomplete 16-bit character
is discarded.

• The comma character, “,” is used in place of the “/” character in the base64
alphabet.

For example, the word “Resume” with both "e"s being the e-acute character, U+0x00e9, is
encoded as “R&AOk-sum&AOk-” (so a folder of that name would be a maildir subdirectory
called “.R&AOk-sum&AOk-”).

Other maildir contents
Software that uses maildirs may also create additional files besides the tmp, new, and
cur subdirectories -- in the main maildir or a subfolder -- for its own purposes.

Messages
E-mail messages are stored in separate, individual files, one E-mail message per file. The
tmp subdirectory temporarily stores E-mail messages that are in the process of being
delivered to this maildir. tmp may also store other kinds of temporary files, as long as
they are created in the same way that message files are created in tmp. The new subdirectory
stores messages that have been delivered to this maildir, but have not yet been seen by any
mail application. The cur subdirectory stores messages that have already been seen by mail
applications.

Adding new mail to maildirs
The following process delivers a new message to the maildir:

A new unique filename is created using one of two possible forms: “time.MusecPpid.host”, or
“time.MusecPpid_unique.host”. “time” and “usec” is the current system time, obtained from
gettimeofday(2). “pid” is the process number of the process that is delivering this message
to the maildir. “host” is the name of the machine where the mail is being delivered. In the
event that the same process creates multiple messages, a suffix unique to each message is
appended to the process id; preferrably an underscore, followed by an increasing counter.
This applies whether messages created by a process are all added to the same, or different,
maildirs. This protocol allows multiple processes running on multiple machines on the same
network to simultaneously create new messages without stomping on each other.

The filename created in the previous step is checked for existence by executing the stat(2)
system call. If stat(2) results in ANYTHING OTHER than the system error ENOENT, the process
must sleep for two seconds, then go back and create another unique filename. This is an extra
step to insure that each new message has a completely unique filename.

Other applications that wish to use tmp for temporary storage should observe the same
protocol (but see READING MAIL FROM MAILDIRS below, because old files in tmp will be
eventually deleted).

If the stat(2) system call returned ENOENT, the process may proceed to create the file in the
tmp subdirectory, and save the entire message in the new file. The message saved MUST NOT
have the “From_” header that is used to mboxes. The message also MUST NOT have any “From_”
lines in the contents of the message prefixed by the “>” character.

When saving the message, the number of bytes returned by the write(2) system call must be
checked, in order to make sure that the complete message has been written out.

After the message is saved, the file descriptor is fstat(2)-ed. The file's device number,
inode number, and the its byte size, are saved. The file is closed and is then immediately
moved/renamed into the new subdirectory. The name of the file in new should be
“time.MusecPpidVdevIino.host,S=cnt”, or “time.MusecPpidVdevIino_unique.host,S=cnt”. “dev” is
the message's device number, “ino” is the message's inode number (from the previous fstat(2)
call); and “cnt” is the message's size, in bytes.

The “,S=cnt” part optimizes the Courier[3] mail server's maildir quota enhancement; it allows
the size of all the mail stored in the maildir to be added up without issuing the stat(2)
system call for each individual message (this can be quite a performance drain with certain
network filesystems).

READING MAIL FROM MAILDIRS
Applications that read mail from maildirs should do it in the following order:

When opening a maildir or a maildir folder, read the tmp subdirectory and delete any files in
there that are at least 36 hours old.

Look for new messages in the new subdirectory. Rename new/filename, as cur/filename:2,info.
Here, info represents the state of the message, and it consists of zero or more boolean flags
chosen from the following: “D” - this is a 'draft' message, “R” - this message has been
replied to, “S” - this message has been viewed (seen), “T” - this message has been marked to
be deleted (trashed), but is not yet removed (messages are removed from maildirs simply by
deleting their file), “F” - this message has been marked by the user, for some purpose. These
flags must be stored in alphabetical order. New messages contain only the :2, suffix, with no
flags, indicating that the messages were not seen, replied, marked, or deleted.

Maildirs may have maximum size quotas defined, but these quotas are purely voluntary. If you
need to implement mandatory quotas, you should use any quota facilities provided by the
underlying filesystem that is used to store the maildirs. The maildir quota enhancement is
designed to be used in certain situations where filesystem-based quotas cannot be used for
some reason. The implementation is designed to avoid the use of any locking. As such, at
certain times the calculated quota may be imprecise, and certain anomalous situations may
result in the maildir actually going over the stated quota. One such situation would be when
applications create messages without updating the quota estimate for the maildir. Eventually
it will be precisely recalculated, but wherever possible new messages should be created in
compliance with the voluntary quota protocol.

The voluntary quota protocol involves some additional procedures that must be followed when
creating or deleting messages within a given maildir or its subfolders. The
deliverquota(8)[4] command is a tiny application that delivers a single message to a maildir
using the voluntary quota protocol, and hopefully it can be used as a measure of last resort.
Alternatively, applications can use the libmaildir.a library to handle all the low-level
dirty details for them. The voluntary quota enhancement is described in the
maildirquota(7)[5] man page.

Maildir Quotas
This is a voluntary mechanism for enforcing "loose" quotas on the maximum sizes of maildirs.
This mechanism is enforced in software, and not by the operating system. Therefore it is only
effective as long as the maildirs themselves are not directly accessible by their users,
since this mechanism is trivially disabled.

If possible, operating system-enforced quotas are preferrable. Where operating system quota
enforcement is not available, or not possible, this voluntary quota enforcement mechanism
might be an acceptable compromise. Since it's enforced in software, all software that
modifies or accesses the maildirs is required to voluntary obey and enforce a quota. The
voluntary quota implementation is flexible enough to allow non quota-aware applications to
also access the maildirs, without any drastic consequences. There will be some non-drastic
consequences, though. Of course, non quota-aware applications will not enforce any defined
quotas. Furthermore, this voluntary maildir quota mechanism works by estimating the current
size of the maildir, with periodic exact recalculation. Obviously non quota-aware maildir
applications will not update the maildir size estimation, so the estimate will be thrown off
for some period of time, until the next recalculation.

This voluntary quota mechanism is designed to be a reasonable compromise between
effectiveness, and performance. The entire purpose of using maildir-based mail storage is to
avoid any kind of locking, and to permit parallel access to mail by multiple applications. In
order to compute the exact size of a maildir, the maildir must be locked somehow to prevent
any modifications while its contents are added up. Obviously something like that defeats the
original purpose of using maildirs, therefore the voluntary quota mechanism does not use
locking, and that's why the current recorded maildir size is always considered to be an
estimate. Regular size recalculations will compensate for any occasional race conditions that
result in the estimate to be thrown off.

A quota for an existing maildir is installed by running maildirmake with the -q option, and
naming an existing maildir. The -q option takes a parameter, quota, which is a
comma-separated list of quota specifications. A quota specification consists of a number
followed by either 'S', indicating the maximum message size in bytes, or 'C', maximum number
of messages. For example:

maildirmake -q 5000000S,1000C ./Maildir

This sets the quota to 5,000,000 bytes or 1000 messages, whichever comes first.

maildirmake -q 1000000S ./Maildir

This sets the quota to 1,000,000 bytes, without limiting the number of messages.

A quota of an existing maildir can be changed by rerunning the maildirmake command with a new
-q option. To delete a quota entirely, delete the Maildir/maildirsize file.