Back up all data.
1. Triple-click anywhere in the line below on this page to select it:
~/Library/Mail/Bundles
Right-click or
control
-click the highlighted line and select
Services ▹ Open
from the contextual menu.* A folder may open, or you may get an error message that the item can't be found. Either result is normal. If the folder does open and has contents, move the contents to the Desktop. Relaunch Mail and test. If there's no change, put the contents of the folder back and quit Mail again.
2. Repeat with this line:
/Library/Mail/Bundles
This time you may be prompted for your login password when you remove the items. Make sure they're removed from the folder and not just copied to the Desktop. If necessary, copy them first and then move the originals to the Trash.
*If you don't see the contextual menu item, copy the selected text to the Clipboard by pressing the key combination command-C. In the Finder, select
Go ▹ Go to Folder...
from the menu bar, paste into the box that opens (command-V). You won't see what you pasted because a line break is included. Press return.