Well written app that works seamlessly with Mail.app, as well as others. I've been using it for nearly a year, and I've had far better luck with this app than any of the built-in solutions or any of the of the commercial products I've tried or been saddled with in a corporate environment.

It just keeps working, and the developer is fantastic. I had an issue that was dealt with by one of the leads personally the same day. Well worth the few bucks to register.