Maildir


The Maildir e-mail format is a common way of storing email messages in which each message is stored in a separate file with a unique name, and each mail folder is a file system directory. The local file system handles file locking as messages are added, moved and deleted. A major design goal of Maildir is to eliminate the need for program code to handle file locking and unlocking.
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Specifications

A Maildir directory usually has three subdirectories named tmp, new, and cur.
The tmp subdirectory temporarily stores e-mail messages that are in the process of being delivered. This subdirectory may also store other kinds of temporary files. The new subdirectory stores messages that have been delivered, but have not yet been seen by any mail application. The cur subdirectory stores messages that have already been seen by mail applications.

Maildir++

Sam Varshavchik, the author of the Courier Mail Server and other software, wrote an extension to the Maildir format called Maildir++ to support subfolders and mail quotas. Maildir++ directories contain subdirectories with names that start with a '.' that are also Maildir++ folders. This extension is not a violation of the Maildir specification, which explicitly provides for the possibility to add more than tmp, new, cur to a maildir.

Technical operation

The program that delivers an email message, a mail delivery agent, writes it to a file in the tmp directory with a unique filename. Circa 1995, the algorithm recommended by Qmail for generating unique filenames was to concatenate, separated by the period character, the current Unix time, the current process identifier, and the current hostname; check whether that filename already exists ; and, if the filename exists, delay delivery for two seconds then re-attempt. By 2000, it was recommended to append, to the PID, the value of a per-process counter whose value should be incremented after each delivery; and the rate-limiting suggestion had been dropped. By 2003, the recommendations had been further amended to require that instead of the PID and counter, the middle part of the filename should be created by "concatenating enough of the following strings to guarantee uniqueness" even in the face of multiple simultaneous deliveries to the same maildir from one or more processes:

  • #n, where n is the output of the operating system's unix_sequencenumber system call, which returns a number that increases by 1 every time it is called, starting from 0 after reboot.
  • Xn, where n is the output of the operating system's unix_bootnumber system call, which reports the number of times that the system has been booted. Together with #, this guarantees uniqueness; unfortunately, most operating systems don't support unix_sequencenumber and unix_bootnumber.
  • Rn, where n is the output of the operating system's unix_cryptorandomnumber system call, or an equivalent source such as /dev/urandom. Unfortunately, some operating systems don't include cryptographic random number generators.
  • In, where n is the UNIX inode number of this file. Unfortunately, inode numbers aren't always available through NFS.
  • Vn, where n is the UNIX device number of this file. Unfortunately, device numbers aren't always available through NFS. and rename to work.)
  • Mn, where n is the microsecond counter from the same gettimeofday used for the left part of the unique name.
  • Pn, where n is the process ID.
  • Qn, where n is the number of deliveries made by this process.

As of November 2018, Bernstein had made no further changes to these filename generation recommendations.
The delivery process stores the message in the maildir by creating and writing to tmp/uniquefilename, and then moving this file to new/uniquefilename. The moving can be done using rename, which is atomic in many systems. Alternatively, it can be done by hard linking the file to new and then unlinking the file from tmp. Any leftover file will eventually be deleted. This sequence guarantees that a maildir-reading program will not see a partially written message. There can be multiple programs reading a maildir at the same time. They range from mail user agents which access the server's file system directly, through Internet Message Access Protocol or Post Office Protocol servers acting on behalf of remote MUAs, to utilities such as biff and rsync, which may or may not be aware of the maildir structure. Readers should never look in tmp.
When a cognizant maildir reading process finds messages in the new directory it must move them to cur. It is just a means to notify the user "you have X new messages". This moving needs to be done using rename, as the non-atomic link then unlink technique may result in duplicated messages. An informational suffix is appended to filenames at this stage. It consists of a colon, a '2', a comma and various flags. The '2' specifies the version of the information that follows the comma. '2' is the only currently officially specified version, '1' being an experimental version. The specification defines flags which show whether the message has been read, deleted and so on: the initial letter of Passed, Replied, Seen, Trashed, Draft, and Flagged. Dovecot uses lowercase letters to match 26 IMAP keywords, which may include standardised keywords, such as $MDNSent, and user defined flags.
Although Maildir was intended to allow lockless usage, in practice some software that uses Maildirs also uses locks, such as Dovecot.

File system compatibility issues

The Maildir standard can only be implemented on systems that accept colons in filenames.
Systems that don't allow colons in filenames can use an alternative separator, such as ";" or "-". It is often trivial to patch free and open-source software to use a different separator.
As there is currently no agreement on what character this alternative separator should be, there can be interoperability difficulties between different Maildir-supporting programs on these systems. However, not all Maildir-related software needs to know what the separator character is, because not all Maildir-related software needs to be able to read or modify the flags of a message ; software that merely delivers to a Maildir, or archives old messages from it based only on date, should work no matter what separator is in use. If only the MUA needs to read or modify message flags, and only one is used, then non-standard alternative separators may be used without interoperability problems.

Software that supports Maildir directly

Mail servers">Mail Transfer Agent">Mail servers