A Mailbox format for unix where the mailbox is a directory and every file in the directory is one email message. Gets around a lot of the serious limitations on the old mbox mailbox format. It's largest drawback is that everyone has their own "index" files that they generate for maildir and there is no agreed upon standard (even within version changes of a single program). In general far superior to mbox mailboxes.
MailDir variants aer used by Cyrus and courier imap daemons.
The details of the MailDir MailBoxFormat? are slightly more complex than other similar schemes (such as MH). MailDir imposes certain naming and structural requirements on it's mailboxes, as follows:
Each MailDir mailbox has three subdirectories: tmp, new and cur. The new subdirectory contains freshly delivered, unread messages. As messages are read, they are renamed into cur. tmp is used to ensure reliable delivery.
MailDir message naming conventions are defined so that message names are unique across shared filesystems, which means you don't get the normal set of file locking issues using most other formats.
Some MetaData, in particular the message flags, is encoded into the filename for messages stored in cur
References (with detailed info):
7 pages link to MailDir:
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http://www.qmail.org/man/man5/maildir.html