The
format for storing
emails used by, among others,
the
MTA qmail written by the
controversial D.J.
Bernstein.
It's main distinguishing feature is that it uses one file per message, unlike e.g. the popular Unix mbox format, which puts many messages into one file.
The main advantage is that locking is generally not necessary, which makes maildir suitable for use over NFS.
More info can be found at
http://cr.yp.to/proto/maildir.html