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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:
>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
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
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
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
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
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,
Urs Janssen <email@example.com>