Apple Mail Plugins are third-party tools that enhance Apple Mail. They are typically installed within the user’s
~/Library/Mail/Bundles/ folder and are launched whenever Mail.app is running. There are many Mail plugins available, many of which are listed on Hawk Wing – Plug-ins for Apple Mail.
The Mail and Messaging frameworks that Mail plugins rely on are private and subject to change by Apple. Since Mac OS X 10.6, plugins have to be updated to support each successive point release. There is no guarantee from Apple that plugins will continue to function as they do today in the future1.
Application plugins are currently prohibited on the Mac App Store. This is one reason why MsgFiler has been rewritten from being a plugin to being a full-fledged application. This is possible because Mail has an extensive AppleScript dictionary that allows much of the functionality of the plugin to exist in a separate application.
1 The MsgFiler 2 Mail Plugin will continue to be updated to support Mac OS X 10.6 and 10.7.