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MMDFII:
A Technical Review
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Douglas P. Kingston III
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Ballistic Research Laboratory
Aberdeen Proving Grounds, Maryland  21005
<dpk@brl>
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The Multi-channel Memo Distribution Facility (MMDF) is one of
the most sophisticated mail systems available for the UNIX\(dg
.FS \(dg
Unix is a trademark of Bell Laboratories.
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operating
system.
MMDF is a mail transport system that supports a variety of
user interfaces and delivery mechanisms. The design was not encumbered
with the need to be compatible with existing mail systems, and as a
result MMDF has a unified family of mail handling programs.  This review
will discuss MMDF's design and operation, concentrating on those features
that are unique to MMDFII, the latest release of MMDF.
.PP
MMDF's design allows it to grow from a
single-host system to a large mail relay without degradation of mail
system performance, and to degrade gracefully as the load becomes huge.
The demands of a high volume mail relay have led to many of MMDF's
innovative design choices.
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Unlike some other systems, MMDF has separate processes for
mail submission and delivery.
Recent changes to the delivery software to permit intelligent
retry strategies based on the retry history for each dead host will
be explained.
The effect of the new domain server
mechanism on address validation will be discussed.
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The separation of mail into channels is key to MMDF's ability to
handle large amounts of mail. Each channel represents a different class
of delivery and each channel has its own queue.  This isolates
problems and allows one to
provide different ``levels of service'' to different channels.
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Other topics to be discussed will include available user
interfaces, the mailing list processor, aliasing, runtime
configuration, and domain based naming.
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The MMDF system was originally developed at the University of Delaware
and has since seen significant development work at the Ballistic
Research Laboratory and University College London.
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