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.

The main advantage is less reliance on file locking that can cause problems (eg some apps use flock(2) while some use fcntl(2), and NFS may or may not support locking). Another advantage is that corruption in a single file doesn't take out hundreds or thousands of messages.

Its 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). Another drawback is it can place a greater burden on the filesystem if you are the type of person who keeps 20,000 messages in a folder. Some filesystems are more efficient than other.

In general far superior to mbox mailboxes.

MailDir variants are used by Cyrus and courier imap daemons. Other mail transfer agents (MTAs) have maildir support, such as exim(1)? and procmail(1) (which is not really an MTA). Also, you'll need a mail client that understands the format, such as Evolution or Mutt.

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):

1. http://www.qmail.org/man/man5/maildir.html
2. http://cr.yp.to/proto/maildir.html