i.Scribe
A lightweight crossplatform email client, supporting POP3, IMAP4 (fetch), [E]SMTP, SOCKS5, and MIME.

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. Features embedded python interpreter for powerful scripting with full object-oriented access to internal structures and classes.

Mozilla Mail/News
Technical information and project documentation of the integrated email and news client in the Mozilla Suite.

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.

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.

SimpleMail Homepage
Portable e-mail client developed for Amiga.