Nmh is an email system based on the MH email system and is intended to be a (mostly) compatible drop-in replacement for MH. Nmh isn't a single comprehensive program. Instead, it consists of a number of fairly simple single-purpose programs for sending, receiving, saving, retrieving and otherwise manipulating email messages. You can freely intersperse nmh commands with other shell commands or write custom scripts which utilize nmh commands. If you want to use nmh as a true email user agent, you'll want to also install exmh to provide a user interface for it - nmh only has a command line interface.