mmdf(5)                          User Manuals                          mmdf(5)

       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 RFC5322, followed  by  a  postmark.  The
       file  format is line-oriented. Lines are separated by line feed charac-
       ters (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) 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 mes-
       sages have already been read.

       Since MMDF files are frequently accessed by multiple programs in paral-
       lel, MMDF files should generally not be accessed without locking.

       Three different locking mechanisms (and combinations  thereof)  are  in
       general use:

       o      fcntl(2)  locking is mostly used on recent, POSIX-compliant sys-
              tems. 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'

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

       o      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 re-
              sides. 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 ob-
       tained. When one of the individual locking methods fails,  an  applica-
       tion 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), RFC4155,

       Urs Janssen <>

Unix                          November 5th, 2013                       mmdf(5)

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