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mmdf(5) 			  User Manuals				 mmdf(5)

NAME
     MMDF - Multi-channel Memorandum Distribution Facility mailbox format

DESCRIPTION
     This  document describes the MMDF mailbox format used by some MTAs and MUAs
     (i.e.  scomail(1)) to store mail messages locally.

     An MMDF mailbox is a text file containing an  arbitrary  number  of  e-mail
     messages.	 Each message consists of a postmark, followed by an e-mail mes-
     sage formatted according to RFC5322, followed by a postmark. The file  for-
     mat  is  line-oriented.  Lines are separated by line feed characters (ASCII
     10).  A postmark line consists of the four characters "^A^A^A^A"  (Control-
     A; ASCII 1).

     Example of a MMDF mailbox holding two mails:

	    ^A^A^A^A
	    From: example@example.com
	    To: example@example.org
	    Subject: test

	    From what I learned about the MMDF-format:
	    ...
	    ^A^A^A^A
	    ^A^A^A^A
	    From: example@example.com
	    To: example@example.org
	    Subject: test 2

	    bar
	    ^A^A^A^A

     In contrast to most other single file mailbox formats like MBOXO and MBOXRD
     (see  mbox(5)  and RFC4155) there is no need to quote/dequote "From "-lines
     in MMDF mailboxes as such lines have no special meaning in this format.

     If the modification-time (usually determined via  stat(2))  of  a	nonempty
     mailbox  file  is	greater than the access-time the file has new mail. Many
     MUAs place a Status: header in each message to indicate which messages have
     already been read.

LOCKING
     Since MMDF files are frequently accessed by multiple programs in  parallel,
     MMDF files should generally not be accessed without locking.

     Three  different  locking mechanisms (and combinations thereof) are in gen-
     eral use:

     *	    fcntl(2) locking is mostly used on recent, POSIX-compliant	systems.
	    Use  of  this  locking  method  is, in particular, advisable if MMDF
	    files are accessed through the Network File System (NFS),  since  it
	    seems the only way to reliably invalidate NFS clients' caches.

     *	    flock(2) locking is mostly used on BSD-based systems.

     *	    Dotlocking is used on all kinds of systems. In order to lock an MMDF
	    file  named  folder,  an  application first creates a temporary file
	    with a unique name in the directory in which the folder resides. The
	    application then tries to use the link(2) system call  to  create  a
	    hard  link	named  folder.lock to the temporary file. The success of
	    the link(2)  system  call  should  be  additionally  verified  using
	    stat(2) calls. If the link has succeeded, the mail folder is consid-
	    ered dotlocked. The temporary file can then safely be unlinked.

	    In	order  to  release  the  lock,	an  application just unlinks the
	    folder.lock file.

     If multiple methods are combined, implementers should make sure to use  the
     non-blocking variants of the fcntl(2) and flock(2) system calls in order to
     avoid deadlocks.

     If  multiple  methods  are combined, an MMDF file must not be considered to
     have been successfully locked before all individual  locks  were  obtained.
     When one of the individual locking methods fails, an application should re-
     lease  all  locks	it acquired successfully, and restart the entire locking
     procedure from the beginning, after a suitable delay.

     The locking mechanism used on a particular system is a matter of local pol-
     icy, and should be consistently used by all applications installed  on  the
     system  which  access MMDF files. Failure to do so may result in loss of e-
     mail data, and in corrupted MMDF files.

CONFORMING TO
     MMDF is not part of any currently supported standard.

HISTORY
     MMDF was developed at the University of Delaware by Dave Crocker.

SEE ALSO
     scomail(1), fcntl(2), flock(2), link(2), stat(2), mbox(5), RFC4155, RFC5322

AUTHOR
     Urs Janssen <urs@tin.org>

Unix			       November 5th, 2013			 mmdf(5)
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