MHonArc A mail-to-HTML converter. Released under the GPL(v2) license.
Pegasus Mail
mutt
Pine (development halted), Pine_(email_client) Pine+OpenSSL HOWTO.
Alpine based on Pine. Patches for Alpine for the latest version. Alpine_(email_client)

MIMEDefang is an e-mail filtering tool that works with the Sendmail “Milter” library. MIMEDefang lets you express your filtering policies in Perl rather than C, making it quick and easy to filter or manipulate your mail.