xmh was born in the summer of 1986, at the Western Software Labs (WSL) of Digital Equipment Corporation in Palo Alto, California. Smokey Wallace, the head of WSL, was working on the early ancestor of the Xt toolkit and the Athena Widgets that are part of X11R5. He was assisted by Terry Weissman, a Stanford graduate student working at WSL for the summer. X11 was still being designed and implemented; all development took place on X10.
The toolkit needed to have its design proven by using it to develop a significant application. At the same time, people at WSL were unhappy with the tools they had for reading mail. The solution to both problems was to have Terry create a toolkit-based mail application.
To prove that the toolkit was useful for developing user interfaces and not merely for building a new mail system, the xmh application was built on top of the MH mail system. Terry also had considerable help on the design of the user interface from Phil Karlton, who (in a previous job at Xerox) had developed a windowing mail user interface named Hardy. (Hardy, of course, was based on an earlier program named Laurel, whose authors never forgave him for naming it "Hardy.")
By the end of the summer, a usable but limited prototype was finished. Terry became a full-time WSL employee and continued working on xmh for about a year. It was ported many times to improved versions of the toolkit. It continued to prove an important test case for the toolkit and later the original X11 sample server.
Since then, Donna Converse has updated xmh further, with supporting contributions by her colleagues on the staff of the X Consortium.