Please follow these instructions to delete the Mail "sandbox" folder.
Back up all data.
Triple-click the line below to select it, then copy it to the Clipboard (command-C):
In the Finder, select
Go▹Go to Folder
from the menu bar. Paste (command-V) into the box that opens, then press return. A Finder window should open with a folder named "com.apple.mail" selected. If it does, move the selected folder — not just its contents — to the Desktop. Leave the Finder window open for now.
Relaunch Mail and test. If the problem is resolved, you'll have to recreate some of your Mail settings. You can then delete the folder you moved and close the Finder window. If you still have the problem, quit Mail again and put the folder back where it was, overwriting the one that may have been created in its place. Post your results.
Caution: If you change any of the contents of the sandbox, but leave the folder itself in place, Mail may crash or not launch at all. Deleting the whole sandbox will cause it to be rebuilt automatically.