I have been using Mozilla Thunderbird for a year or so, and it has worked really well for me. I get a lot of email (and I do my best to respond to it, even if it takes me a while).
One feature that I really like is the ability to color-code individual messages in my inbox at the touch of a key. Pressing “1″ makes the message header red, “2″ is orange, “3″ is green, “4″ is blue, and “5″ is purple. The “0″ key returns the item to its normal color.
When an especially urgent email comes in and I want to be sure to respond, I can simply hit “1″. You can see my Inbox in the picture. I am also trying to use blue for items that I want to post to my blog. The other colors are more or less random at this point, but I could assign meanings to them if I wanted to.
I’m a big believer in keyboard shortcuts, and I am a big believer in making use of all interesting application features, not just the obvious ones. I do read and re-read user guides and FAQs from time to time in search of handy little things like this.