Mutt supports reading and writing of four different mailbox formats: mbox, MMDF, MH and Maildir. The mailbox type is autodetected, so there is no need to use a flag for different mailbox types. When creating new mailboxes, Mutt uses the default specified with the $mbox_type variable.
mbox. This is the most widely used mailbox format for UNIX. All messages are stored in a single file. Each message has a line of the form:
From me@cs.hmc.edu Fri, 11 Apr 1997 11:44:56 PST
to denote the start of a new message (this is often referred to as the ``From_'' line).
MMDF. This is a variant of the mbox format. Each message is surrounded by lines containing ``ˆAˆAˆAˆA'' (four control-A's).
MH. A radical departure from mbox and MMDF, a mailbox
consists of a directory and each message is stored in a separate file.
The filename indicates the message number (however, this is may not
correspond to the message number Mutt displays). Deleted messages are
renamed with a comma (,) prepended to the filename. Note: Mutt
detects this type of mailbox by looking for either .mh_sequences
or .xmhcache (needed to distinguish normal directories from MH
mailboxes).
Maildir. The newest of the mailbox formats, used by the Qmail MTA (a replacement for sendmail). Similar to MH, except that it adds three subdirectories of the mailbox: tmp, new and cur. Filenames for the messages are chosen in such a way they are unique, even when two programs are writing the mailbox over NFS, which means that no file locking is needed.