bionic (5) maildir.maildrop.5.gz

Provided by: maildrop_2.9.3-1build1_amd64 bug

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.

SEE ALSO

       maildirmake(1)[1].

AUTHOR

       Sam Varshavchik
           Author

NOTES

        1. maildirmake(1)
           http://www.courier-mta.org/maildirmake.html

        2. RFC 1521
           http://www.rfc-editor.org/rfc/rfc1521.txt

        3. Courier
           http://www.courier-mta.org

        4. deliverquota(8)
           http://www.courier-mta.org/deliverquota.html

        5. maildirquota(7)
           http://www.courier-mta.org/maildirquota.html