PMMail 2000  
   Support for multiple accounts, email accounts are presented in an 'Explorer' type file tree. Features full strength PGP for security with a license for international export. For Windows and OS/2.
   http://www.pmmail2000.com/

Mulberry  
   Mail client for POP3 and IMAP on Windows and Macintosh platforms with support for remote administration and enhanced security.
   http://www.cyrusoft.com/mulberry/mulbinfo.html

Mahogany  
   Cross-platform mail client, formerly known as M; available for Unix/X11 and, in a preview version, for MS Windows. Full support for MIME, SMTP, POP3, IMAP, NNTP, X-Faces and several different mail folder file formats. Full internationalisation 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/

SimpleMail Homepage  
   Portable e-mail client developed for Amiga.
   http://simplemail.sourceforge.net/index.php

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

eMailman - AOL Alternatives  
   Alternative AOL email clients.
   http://www.emailman.com/aol/

OpenSmtp.Net  
   An SMTP component written in C#. It extend the BCL Mail classes, allowing developers to use SMTP servers other than MS SMTP. It also has an extendable web service that allows the sending of email via HTTP/SOAP. [BSD License.] Windows only.
   http://sourceforge.net/projects/opensmtp-net/