is a powerful email client based on Mozilla's Thunderbird, and, like Thunderbird, it now supports extensions. The selection of add-ons is limited right now, but it includes some useful ones: ReminderFox handles reminders and to-do items. Minimize to Tray is just what it sounds like, allowing Windows users to put Postbox in the system tray. Nostalgy adds keyboard shortcuts, and MozBackup backs up your messages and settings. This is a good start, but it'll interesting to see what other useful extensions pop up now that the door is open.
The latest build of Postbox also adds a long list of other useful features. If you're switching from Mail.app, you can now easily migrate your settings to Postbox. Postbox will now also pull photos for your contacts from OS X's address book or from Twitter. Several performance upgrades and fixes of annoying issues from earlier versions are also in place now, so it looks like a good time to consider giving Postbox a try.