Elm is a text-based email client for Unix systems. The name elm originated from the phrase ELectronic Mail.

The MH Message Handling System is a free, open source e-mail client. It is made from several different programs which are designed to work from the command line.

Mutt is a mail user agent for Unix and Unix like systems. It's text based and has a lot of powerful features. People that use Mutt are told mutters. They think that Mutt is the best mail client.

procmail is a mail delivery agent.

Graphical email clients based on the X Windows protocol (sometimes referred to as X or X11).