The MH Message Handling System is a free, open source e-mail client.
MH was proposed by R. Stockton Gaines and Norman Shapiro and developed by Bruce S. Borden and others at RAND Corporation.
Subsequently development was taken over by Marshall T. Rose and John L. Romine working at University of California, Irvine.
It was forked from version MH 6.8.3 by Richard Coleman while working at the Georgia Institute of Technology, and released under the BSD License,.
It incorporates the "LBL changes" made to MH in the late 80s by Van Jacobson, Mike Karels and Craig Leres.
The maildir storage format made popular by qmail and the Courier Mail Server improves upon several features first shown in MH: a separate file per message, a separate directory per mail "folder," and exploitation of hardlinks and other advanced filesystem features for improved performance in space and time.