Bring up a terminal and check to see if it's still running. In the terminal type:
ps -ef | grep Mail | grep App
If you get anything returned, it will look like this:
501 5152 133 0 7:09AM ?? 0:07.54 /Applications/Mail.app/Contents/MacOS/Mail -psn_0_3949508
The process id (pid) is the 2nd field on the information returned, has been highlighted in the line above, and it's what you want to kill:
kill 5152
Once done run the "ps" command again. If that returns information again, change the "kill pid" to "kill -9 pid".
$0.02 worth.