Hi,
the problem is : the upgrade of Mail (4.5 to 4.6) failed.
To check this, select Mail 4.6 in your Applications folder, Cmd + i (get infos).
If the size is 8,1 Mo (instead of 31,5 Mo), trash this file.
(it's the Mail update only, not Mail itself).
Then, check that Mail 4.5 is still here, in the /Applications folder and did not get renamed.
(if it is nowhere, see last line).
If yes, apply this update :
http://support.apple.com/kb/DL1586?viewlocale=en_US&locale=en_US
(this is the update that also updates Mail from 4.5 to 4.6)
After applying the update, Mail will be in 4.6 version, weigh 31.5 Mo, and will work.
If you fail, I will give you a link to Mail 4.6 which I have uploaded.