maildir(5) - phpMan

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:

o The "=" padding character is omitted. When decoding, an incomplete 16-bit
character is discarded.

o 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.