Mahogany
   An OpenSource cross-platform mail and news client available for X11/Unix and MS Win32 platforms, supporting SMTP, POP3, IMAP, NNTP, and full MIME support.
   http://mahogany.sourceforge.net/

Mozilla Mail/News
   Technical information and project documentation of the integrated email and news client in the Mozilla Suite.
   http://www.mozilla.org/mailnews/

OpenSmtp.Net
   An SMTP component written in C#. Allows developers to use SMTP servers other than MS SMTP and has a web service that allows the sending of email via HTTP/SOAP. [BSD License.] Windows only.
   http://sourceforge.net/projects/opensmtp-net/

SimpleMail
   A new mailer for the Amiga platform (both classic and OS4) and compatible systems (MorphOS).
   http://simplemail.sourceforge.net/index.php

eMailman
   Enables using third party e-mail programs to receive or send AOL mail.
   http://www.emailman.com/aol/

i.Scribe/InScribe
   A lightweight crossplatform email client, supporting POP3, IMAP4 (fetch), [E]SMTP, SOCKS5, and MIME with an intergrated contact database and calendar.
   http://www.memecode.com/scribe.php