honestly? Thunderbird, but sometimes I'm tempted to give it up and use just a web app or Opera Mail. Why I use it? Proprietary software smells and it is the best in security terms (that's why it's so much popular in Germany).

It has a bad integration with gmail/outlook for students (I know it's their problem). It doesn't have a modern UI (things like a multi line view that would make users happier). Furthermore, I can't use it in background since it takes 300+ MB. I may list something else, but these are things that could be solved with a more active development.

What is better? I would use mailpile (it promises a lot!), but web apps doesn't give you so much freedom.