mmdf(5)               (February 18th, 2002)               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 num-
          ber of e-mail messages.  Each message consists of a post-
          mark, followed by an e-mail message formatted according to
          RFC822 / RFC2822, followed by a postmark. The file format 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)) 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 pro-
          grams in parallel, MMDF files should generally not be
          accessed without locking.

          Three different locking mechanisms (and combinations

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     mmdf(5)               (February 18th, 2002)               mmdf(5)

          thereof) are in general use:

          +o    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.

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

          If multiple methods are combined, implementors 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 indi-
          vidual locks were obtained. When one of the individual lock-
          ing methods fails, an application should release 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 mat-
          ter of local policy, 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),
          RFC822, RFC2822

     AUTHOR
          Urs Janssen <urs@tin.org>

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