Sometimes Mail seems to be stuck (especially when you are trying to quit it). Many times, people force-quit Mail because they think it’s frozen, when actually Mail is busy synchronizing folders, or getting mail, or otherwise doing what it’s supposed to be doing. You’ll learn a lot about Mail by showing the Activity window (in Mail, go to the Window menu, then to Activity).
Leave the Activity window open in a corner of your screen and keep an eye on it as you use the Mail program. I think you’ll find that Mail is quite the busy bee, and when you can see the program working you’ll be much less likely to give up on it and force-quit. I don’t leave Activity up all the time, but when things seem slow I look to Activity to see what’s going on.