A bizarre, inexplicable problem hit me this morning. I suddenly had no Mail.app. Everything appeared fine, and I even managed to answer one email. The rest of my emails gave me some message about needing to be downloaded from the server (something I have never seen before). I did some digging and found this awesome trick to use when encountering similar Mail.app problems.
Rename it to:
Restart Mail and let it reload your messages. All was fine after that!