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 OS 7 and 8 only; no longer maintained or supported.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.
Collaboration-world - 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.
Entourage by Microsoft
Offers integrated e-mail, address book, calendar, and newsgroup reader. Includes feature description, documentation, upgrade support, and trial download.
envelope v1.0
envelope v1.0 is a Macintosh application that adds single-click encryption to your Eudora email.
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, suppor…
Outlook Express
Offers newsgroup access, multiple accounts, and Exchange Server compatibility. OS 9 and earlier only; no longer maintained or supported. Includes free download.