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The Maildir e-mail format is a common way of storing e-mail messages, where each message is kept in a separate file with a unique name, and each folder is a directory. The local filesystem handles file locking as messages are added, moved and deleted. A major design goal of Maildir is to eliminate program code having to handle locking, which is often difficult.
Contents
Specifications
A Maildir directory (often named Maildir) usually has three subdirectories named tmp, new, and cur.
Maildir
The original Maildir specification was written by Daniel J. Bernstein, the author of qmail, djbdns, and other software. Although the original specification was written specifically for Bernstein's qmail, it is general enough to be implemented in many programs.
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 '.' (dot) 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. Algorithms for generating unique filenames can combine time, host name, and a number of pseudo-random parameters to ensure uniqueness, taking into account that multiple messages can be delivered at the same time.
The delivery process stores the message in the maildir by creating and writing to tmp/unique, and then moving this file to new/unique. 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 (MUAs) which access the server's file system directly, through Internet Message Access Protocol or Post Office Protocol servers acting on behalf or 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 (either a POP or IMAP server, or a mail user agent acting locally) 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 (to separate the unique part of the filename from the actual information), 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 (capital) 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.
Filesystem Compatibility Issues
The Maildir standard can only be implemented on systems that accept colons in filenames.
Systems that don't allow colons in filenames (This includes Microsoft Windows and some configurations of Novell Storage Services.) 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 ("read", "replied to" etc.); 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.