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The MH Message Handling System is a free, open source e-mail client. It is different from almost all other mail reading systems in that, instead of a single program, it is made from several different programs which are designed to work from the command line provided by the shell on Unix-like operating systems. Another difference is that rather than storing multiple messages in a single file, messages each have their own separate file in a special directory. Taken together, these design choices mean that it is very easy and natural to script actions on mail messages using the normal shell scripting tools. A descendant of MH continues to be developed under the name of nmh.

Designed with the Unix philosophy in mind, MH is composed of separate programs such as show, to view a message, scan, to see message titles and rmm to remove messages. By using the pick program, it is possible to select messages, based on sender for example, which the other programs act on.

Because the different programs are run separately and at different times, communication between them has to be arranged specially. Information such as the mail which is currently selected is stored in files (in this case by an .mh_sequences file in the user's MH directory tree).

MH was proposed by R. Stockton Gaines and Norman Shapiro and developed by Bruce S. Borden and others at RAND Corporation. The initial version of MH had been created by 1979. Subsequently development was taken over by Marshall T. Rose and John L. Romine working at University of California, Irvine. The last release of MH was 6.8.4 which was a maintenance patch for the 6.8.3 release.

nmh is the current active version of MH. It was forked from version MH 6.8.3 by Richard Coleman while working at the Georgia Institute of Technology. It actually removed some features such as the daemon but also improved others such as portability and MIME support. Earlier versions of nmh known as "LBL" changes were made in the late 80s by Van Jacobson, Mike Karels and Craig Leres. The most recent release of nmh is version 1.6.

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