The latest full releases of MH, nmh, or GNU mailutils, which includes their source code, are available via the following links:
To get exmh, please see the Section Getting exmh.
You can read about how to get MH-E by reading the Section Getting MH-E in The MH-E Manual. xmh is a standard part of the X Window System; you can get it from http://www.x.org/.