This project has moved to <http://christianserving.org/project/truepreview>. TruePreview is a plugin for Apple's Mail.app that allows you to truly preview messages.
Advanced email suite.
A plugin for MacOS X's Mail.app application that allows access to a Google Gmail account
Ginko - A mail and news client for OpenStep platforms. Initial targets are Mac OS X and GNUstep. Ginko will provide a unified view on mail and news while being optimized for 'disconnected' work.
A plugin for the MacOS X Mail application that adds the ability to fetch mail stored on a httpmail (Hotmail) server
Handles GetURL apple events, like a mail client would handle a mailto link, by delegating them to the mutt mail client, using AppleScript and Terminal.app. Both the developers are viewing this mostly as a learning experience, so you are welcome to join.
RSS4Mail is a RSS news reader in the form of a bundle for Mac OS X\'s Mail.app
Periodically check Yahoo! mail accounts for new mail on Mac OS X.
Binary package of Ximian (Novell) Evolution (X11 version) for Mac OS X, with native Mac OS X installer.