Mutt is a text-based email client for UNIX-based operating systems. To say it is highly configurable is an understatement. It can be compiled to support a variety of mail protocols and runs in almost any terminal emulator. Unfortunately, the glory days of Mutt have come and gone. In the days before Gmail had made web mail available to everyone, Mutt was a great option for any UNIX administrator that might need to use multiple computers. Since Mutt was running directly on the mail server, it didn't matter what type of client you were using, as long as you had some sort of SSH client available, you could check your mail.
Now, you are much more likely to see an administrator using their web browser and some form of web mail. Gmail even has keyboard shortcuts to make the most musophobic UNIX administrator happy.
Even so, I still use Mutt on a regular basis for things like checking
root mail, or when my fingers get tired of the track pad on my PowerBook G4.
Mutt has an active community and is still under development. You can find all the information you've ever wanted on Mutt's home page.