Mail User Agent (MUA) - a software used for reading and writing emails. MUA is also called Email Client.
Example of MUA:
Mutt - text-based, supports MIME, GPG, PGP and threading
?Alpine - text-based, friendly, powerful
Icedove (Mozilla Thunderbird)
?Sylpheed (Light weight, GTK+)
Remote-mail retrieval and forwarding utility: