Postbox has a MASSIVE amount of features for a mailing app, so many that I would easily say it rivals Outlook in its abilities to act as not just a mail program but a full contact manager. GMail users especially (like myself) will enjoy how familiar the interface is, and how many of GMail's Labs features are built right into Postbox. They also have updates fast and frequent.

Beautiful UI, with some great searching and indexing features. IF you want offline access to email with some high-powered search & sort features, it's a great solution.

I've tried them all, Postbox is not the most beautiful but it is the most efficient. I can move mail around in a flash, find it even quicker. I can sort it every which way and there are so many ways to categorize and find it that I can take my inbox from 2000 messages to 100 quickly. This is extremely important for someone that gets a huge amount of mail.