Maildir

Maildir is a mail storage format developed by D. J. Bernstein for qmail. In Maildir, every message is a file. Every logical mail folder in Maildir consists of 3 directories in file system - "tmp", "new" and "cur". These 3 directories correspond to 3 stages a message undergoes as it arrives:
1. When a message is being downloaded, it is saved into "tmp".
2. When download finished, the message is moved (hard-linked) into "new". Until this moment the message is considered non-existent.
3. Messages in "cur" are just like messages in "new". The difference is that those in "cur" are no longer new mail: they have been seen by the user's mail-reading program (the message gets flags representing it's state: read/unread starred, flagged, etc.)

Useful links:
http://cr.yp.to/proto/maildir.html
http://en.wikipedia.org/wiki/Maildir
http://wiki.dovecot.org/MailboxFormat/Maildir
http://www.courier-mta.org/maildir.html
http://wiki.mutt.org/?ExtendedMaildirFormat