Problem with Mail.app

[lordthistle]

Every once in a while I cannot quit Mail: it refuses to quit for allowing me to restart the computer.

So today I had to force quit Mail.app and reboot.

After rebooting, Mail.app:
- does not show any eMail. Everything is empty;
- it refuses to quit again.

I am running OS X 10.8.5 with Mail.app version 6.6.

I have looked for a solution or a hint, but could not find anything.

Is there anyone who can help me (besides Apple that is usually not very helpful)?

-- thistle