I use this because is the simple, fast an stable email client, of course i hope the planned features ;).

Out of all the email programs i use (and support), Gyazmail is the most reliable and flexible to use. Sure it lacks some features like spotlight integration or encrypted emails (which i would really like), but what it does, it does very well. Very configurable, very stable, very clean. Also uses unix mbox files, rather than large corruptable databases.

Started using this because it was faster than Apple's Mail app and was designed to integrate with SpamSieve. It remains my primary email client.