I've had quite a few problems with the mail feature recently:
1. Emails refusing to send. It stalls for five minutes, and then I receive a notice that a copy has been placed in my outbox. This just happened today and I tried restarting and rebooting as suggested in other threads, but the message still didn't send. I can tell this wasn't a bigger problem with my mail client since I managed to get the content on my PC through dropbox and copy it into a new message, which did send, and I could read my new mail fine. (usually when I have these problems, I can save a draft and send it from another computer, but today I couldn't even do that.)
2. I tried to delete that message that wouldn't send from my outbox, once I'd sent a copy through other means, but it kept reappearing.
3. Emails going missing after I move them to a different folder. They don't show up in Trash, they just vanish and I never see them again. Additionally, sometimes the move seems to fail-- the message reappears in its original folder, or I get an error message. And sometimes "unsent" mails eventually send (i know because I get replies), and sometimes the recipient says they even got multiple copies, but these mails never show in my sent folder so I lose them.
4. This is more of a minor annoyance, but at times the text of an e-mail in the reply window becomes oddly formatted, so that I have to scroll horizontally and can't see all of what I'm typing.