MMDF, the Multichannel Memorandum Distribution Facility, is amessage transfer agent (MTA), a computer program designed to transmitemail.
MMDF was originally developed at theUniversity of Delaware in the late 1970s, and provided the initial means of operatingCSNET, the predecessor toNSFNET.[1][2] It grew in popularity throughout the 1980s, and was selected by theSanta Cruz Operation as the MTA it would distribute with SCO UNIX in 1989. It was also adopted as the basis for other commercial efforts, including the gateway used to connect theMCI Mail service to Internet mail. A re-coded variant of MMDF, calledPascal MDF (PMDF) was written at theUniversity of Pennsylvania forVMS and was eventually commercialized throughNed Freed's Innosoft, which subsequently ported PMDF toTru64 Unix andSolaris. In 1999 PMDF was translated fromPascal toC. The C version of PMDF became the basis of theSun Java System Messaging Server ofSun Microsystems, while rights to PMDF itself were purchased by Process Software, which then ported PMDF toLinux.[3]
As its name denotes, MMDF is an MTA oriented around the idea of channels. Each means of formatting and transporting mail into or out of the mail system is a channel, and is implemented by a separate executable. This makes MMDF a highly modular system, with each module having all of the idiosyncratic syntax and semantic information necessary for a particular email technology or network, as well as theleast privilege necessary, with the authority of each module partitioned from others. An inbound channel receives messages (via the protocol and in the format it implements) and an outbound channel delivers messages (via its relevant protocol and mapping into the relevant format). Internally, MMDF uses a canonical representation for message content and header, including addresses.
Some examples of MMDF channels areSMTP,UUCP, and local (for delivering mail into local mailboxes and accepting mail submitted on the local system). MMDF was used on theCSNET network.
A message that flows through MMDF will typically follow this path:
MMDF approaches administrative configuration differently than other popular MTAs. In the choice between placing specialized knowledge into the software, versus requiring that it be created through administrator's configuration instructions, MMDF chose the former. Hence, arbitrary header rewriting is performed by hard-coded software, with configuration limited to choices among existing rewriting alternatives. This makes configuration simpler and safer for administrators, who use simple key-value textual tables. It also takes more effort to create a new rewriting choice, but that effort needs to occur only one time, by a single technical expert.
The main types of tables are domain, channel, and alias tables.
DNS can be and usually is used for these purposes as well, in the form of "DNS tables" that have the same key-value form. The meaning and effect of entries in these tables are more obvious than the configuration data of more generalized MTAs, but their restricted form also limits the effects that can be produced.
By the end of 2006, MMDF had a market share well behind other mailers such assendmail,postfix,qmail,exim, andMicrosoft Exchange.[4] MMDF is consideredlegacy software.