MailTags

With MailTags installed, you can add comments, due dates, projects and priority to your messages right inside your mail viewer window.

You can then use this metadata as the basis for Spotlight searches, Smart Mailboxes, and Mail Rules. For example, you can create a Smart Mailbox that will show you all messages in project: "Great Novel" that have a due date in the next three days. MailTags also integrates with Mail Act-On to allow tagging of message via simple keystrokes, or by rules for receiving mail. For example, create an Act-On rule for the d key that will defer the due date of the message for 5 days.