E-mail Clients

Claris Emailer
Offers integration with older versions of AOL, Applelink, and Compuserve. OS 7 and 8 only; no longer maintained or supported. Includes free download.
http://www.mac.org/internet/emailer/

Correo
Project to develop an email client based on the Thunderbird codebase, but with a native MacOS X interface. Includes project news and downloads.
http://nkreeger.com/correo/

Entourage by Microsoft
Offers integrated e-mail, address book, calendar, and newsgroup reader. Includes feature description, documentation, upgrade support, and trial download.
http://www.microsoft.com/mac/products/entourage2004/entourage2004.aspx

Envelope
A Macintosh application that adds single-click encryption to your Eudora email.
http://www.mtt.com/envelope

GNUMail.app
A fully featured mail application running on Linux (or FreeBSD, OpenBSD, etc) and Apple Mac OS X. It uses the GNUstep development framework or Apple Cocoa.
http://www.collaboration-world.com/cgi-bin/project/index.cgi?pid=2

JustMail
Offers e-mail access via any Java enabled browser. Includes feature description, online demo, and trial download.
http://www.mstay.com/jm10_ab1.html

Magellan
Offers multi-lingual e-mail with right-to-left or left-to-right typing, spam filtering, HTML mail, SMTP authentication, auto-forwarding, and some AppleScript support. OS X and OS 9. Includes feature descriptions, FAQs, trial download, and upgrade option.
http://www.makienterprise.com/magellanpro/magellanpro.html

Mailsmith
Barebones Software's mail client with wide variety of features for beginning and power users. For OSX and 9.
http://www.barebones.com/products/mailsmith/index.shtml

Nisus Email
Offers drag-send feature, global window, auto-send-attachment directory, and external word processor compatibility. OS X and OS 8.5 and higher. Includes feature description, support forum, screenshots, and trial download.
http://www.nisus.com/NisusEmail/

Outlook Express 5 for Mac
Offers newsgroup access, multiple accounts, and Exchange Server compatibility. OS 9 and earlier only; no longer maintained or supported. Includes free download.
http://www.microsoft.com/mac/otherproducts/outlookexpress/outlookexpress.aspx?pid=outlookexpress

PowerMail
Offers built-in spam filter and integration with SpamSieve, POP and IMAP support, customizable toolbars, AppleScript support, multiple filter rules, and SMTP authentication. OS X only. Includes feature descriptions, FAQ, testimonials, trial download, and sample AppleScripts.
http://www.ctmdev.com/

QuickMail
Offers POP and IMAP support, SMTP authentication, filter rules, multiple accounts, and spam filter. For OS X and OS 9, as well as Windows XP. Includes documentation, support forums, and trial download.
http://www.outspring.com/

SnapMail
Peer-to-peer messaging and chat utility offers file transfer between Windows and Mac computer, reminder messages, templates, return receipt request, and built-in spell checker. OS X and OS 9, as well as Windows. Includes documentation, FAQ, testimonials, and trial download.
http://www.glassbead.com/snapmail/