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       maildir - E-mail directory




       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

           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

       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.

       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

   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:
       “”, or “”.  “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
       “,S=cnt”, or
       “,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).

       Applications that read mail from maildirs should do it in the following

       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

   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:

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

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

       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.




        1. maildirmake(1)
           [set $man.base.url.for.relative.links]/maildirmake.html

        2. RFC 1521

        3. Courier

        4. deliverquota(8)
           [set $man.base.url.for.relative.links]/deliverquota.html

        5. maildirquota(7)
           [set $man.base.url.for.relative.links]/maildirquota.html