Windows

The Bat!
Features automatic spell checker, mail notification, message templates, Mail Dispatcher for managing messages on server on-line. Support for IMAP4, POP, APOP, SMTP, and PGP.
http://www.ritlabs.com/en/products/thebat/

Becky Internet Mail
A multi-featured E-mail client for Windows 95/98/NT4.0. Features Explorer-like interface, multiple accounts, offline operation, and a powerful text editor.
http://www.rimarts.co.jp/becky.htm

Eureka Email
E-mail client with built-in junk email filtering.
http://www.eureka-email.com

EverDesk
Integrates email and file management in a user activity interface, which allows to create, store, and manage emails, attachments, contacts, web links, pages and other non-email documents side by side in the same activity folders.
http://www.everdesk.com/

Express Plus
E-mail client with mail filters, mail merging for bulk sending personalized messages and support for integration with Time and Chaos contact manager.
http://www.chaossoftware.com/express-plus-email.asp

ExpressIT! 2000
E-mail client software that works under Windows 95/8 and NT with a variety of transports and servers and offers a variety of customization options
http://www.ssimail.com/Expressit2.htm

MailPeek
POP3 client for multiple e-mail accounts.
http://www.mailpeek.de/

Maxiworks E-mail Express
Portable email client (fits on 1 floppy disc). Allows access to email from anywhere in the world using any (compatible) computer connected to the Internet.
http://www.maxiworks.com/

Pegasus Mail
Free mail client for Novell and Winsock TCP/IP by David Harris.
http://www.pmail.com/

Poco Mail
E-mail client for Windows noted for its safety and protection from viruses.
http://www.pocomail.com/

Sendmail for Windows
Send email messages, newsletters, invoices, and more to a mailing list. Features include: Mail Merge and Personalization, ODBC connectivity, Advanced address verification, and multiple file attachments.
http://www.MailListBot.com

SpamEater
Anti-Spam and Email Notification Utility for Windows 95, 98, NT 4.0, and 2000.
http://www.hms.com/

Unixmail for Windows
Unixmail for Windows is a package of unix mail tools ported to MS Windows (win32), and bundled together with an easy-to-use installer.
http://unixmail-w32.sourceforge.net/

Vista NetMail
Full-featured Windows e-mail client designed for e-business.
http://www.internet-soft.com/netmail.htm