nmh

This is the nmh mail user agent (reader/sender), a command-line based mail reader that is powerful and extensible. nmh is an excellent choice for people who receive and process a lot of mail. Unlike most mail user agents, nmh is not a single program, rather it is a set of programs that are run from the shell. This allows the user to utilize the full power of the Unix shell in coordination with nmh. Various front-ends are available, such as mh-e (an emacs mode), xmh, and exmh (X11 clients). nmh was originally based on MH version 6.8.3, and is intended to be a (mostly) compatible drop-in replacement for MH.

mmh

This is the mmh mail user agent (reader/sender), a command-line based mail reader that is powerful and extensible. nmh is an excellent choice for people who receive and process a lot of mail. Unlike most mail user agents, mmh is not a single program, rather it is a set of programs that are run from the shell. This allows the user to utilize the full power of the Unix shell in coordination with mmh. Mmh is a modified version of the electronic mail handling system nmh. Nmh (new MH) itself was originally based on the package MH-6.8.3, and was intended to be a (mostly) compatible drop-in replacement for MH. In contrast, mmh is not intended to be a drop-in replacement for nmh, rather mmh breaks compatibility to nmh in order to modernize and simplify it.