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       MMDF - Multi-channel Memorandum Distribution Facility mailbox format


       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 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:

              Subject: test

              >From what I learned about the MMDF-format:
              Subject: test 2


       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.


       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 general 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 considered 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, 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 individual locks were obtained. When one of the  individual
       locking  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 matter 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.


       MMDF is not part of any currently supported standard.


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


       scomail(1), fcntl(2), flock(2), link(2), stat(2), mbox(5), RFC822, RFC2822


       Urs Janssen <>