Eudora
Dorner, Steve, Technical Information, About.com E-mail Center: Eudora, Allexperts Eudora Q&A, Andrew Starr's [Unofficial] Eudora Site, Eudora Beta Testing Programs, Eudora Books, Eudora Email Software by QUALCOMM, Eudora Filter Organizer, Eudora Resources, Eudora Visual QuickStart Guide News & Tips, eudora-win Mailing List
Java
CraftMail, Grendel, HotJava Views, ICEMail, JVMail, Knife, Mailpuccino, Polarbar Mailer, Pooka, POP3 Mail Viewer on J2ME Devices
Mozilla Thunderbird
Mozilla.org: Thunderbird 1.0 Email Client Has Land, MozillaZine Forums - Thunderbird, Neowin Interview: The Mozilla Thunderbird Develope, The Mozilla Thunderbird Mail Project, The Rumbling Edge, The Rumbling Edge, Thunderbird, Thunderbird Extensions, Thunderbird Help, Thunderbird Themes
Pine
Gopi Sundaram's comp.mail.pine FAQ, Help and Patches for Pine, Holger Lamm's PGP4Pine, Nancy McGough's Page on Pine, OpenSSL Configuration for Pine, Peter Daum's PGP4Pine, Peter Hanecak's PGP4Pine, PGPenvelope, Pine for OpenVMS, Pine for OS/2
Tools
CompuSven Incorporated, E-Mail Shuttle, Faces Archive, Filtering Mail FAQ, HWG Filtering FAQ (Windows, Mac, Unix), Infinite Ink's Robots and Mail Filtering, Information Filtering Resources, InterGuru's E-Mail Address Conversions, Mail2RDF, MailGoGoGo
Unix
ELM, MH, Mutt, Procmail, X, archivemail, BSD Vacation V2, Fetchmail, maildrop - mail delivery agent with filtering, Mailfilt, by Mike Fisk (Unix/Perl), Pygmy, The Fetchmail FAQ, Vacation Auto-Reply (Unix)
Web-Based
Acmemail, Advanced Webmail, Advox Omnigate, AllNetMail, AnyEMail, Atmail, Auto-EmailPro, CascadeSoft W3Mail, Caudium Mail (CAMAS), EMU Mail
Windows
Bulk Mailers, Microsoft Outlook, Microsoft Outlook Express, Tools, Becky Internet Mail, Calypso Email, ElectroMail, Eureka Email, EverDesk, Express Plus, ExpressIT! 2000, Forte Agent, Mailbird, MailPeek
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.
Mulberry
Mail client for POP3 and IMAP on Windows and Macintosh platforms with support for remote administration and enhanced security.
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.