Maildir

Maildir was designed by Daniel J. Bernstein circa 1995, with a major goal of eliminating the need for program code to handle file locking and unlocking through use of the local filesystem.

[1] Maildir design reflects the fact that the only operations valid for an email message is that it be created, deleted or have its status changed in some way.

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:[7] This 2003 algorithm was criticised[8] in 2006 as being unnecessarily complex by Timo Sirainen, the creator of Dovecot.

[9] On modern POSIX systems, temporary files can be safely created with the mkstemp C library function.

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 of remote MUAs, to utilities such as biff and rsync, which may or may not be aware of the maildir structure.

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.

[11] This moving needs to be done using the atomic filesystem rename(), as the alternative link-then-unlink technique is non-atomic and may result in duplicated messages.

[15] Systems that don't allow colons in filenames (this includes Microsoft Windows and some configurations of Novell Storage Services) can use a non-standard alternative separator, such as ";" or "-".