Mail Act-On combines keyboard shortcuts and message rules to unleash a lot of unexpected potential. For that purpose, Mail Act-On adds a new tab to Mail's "Rules" setup that works just like and just as handily as the standard rules (now appearing as "Inbox Rules"), but lets you assign keyboard shortcuts. While in the Rules setup, note the new "Outbox Rules", which has Mail Act-On apply actions automatically after you've sent a message.

"Ctrl" plus your custom key immediately applies the desired action to any group of messages. Mail Act-On also offers a control panel as an alternative, which lets you pick actions on the screen and offers swift shortcuts to moving and copying messages (when picking a folder, Act-On assists with auto-completion). You can set up a Mail Act-On action that moves a message to a folder, for example, or gives it a color background to make it stand out.

Of course, you can also make a Mail Act-On action do more than one thing (de-flag and archive a message, for example). You can even use the criteria to apply certain actions to particular messages. Set up two Mail Act-On actions for the key "w", for example, one that moves any message to the "Action" folder and another that colors the message red if the sender is your boss.

Unfortunately, creating and maintaining such more complex criterion-based actions can develop into a challenging exercise for the mind, and Mail Act-On does not support you much in getting the setup right. If something goes wrong, take heart in Mail Act-On's handy undo function. Speaking of something going wrong, I could not get Act-On to compose replies with its actions.