Add following two lines to Info.plist:
You can find the Info.plist in:
If you already opened Mail (and received the disable warning), it’s probably in a folder named “Bundles (Disabled)”; just move the content to Bundles again.
You can do the same ‘hack’ with GPGMail; it’s located at
(~ or /)Library/Mail/Bundles/GPGMail.mailbundle/Contents/
These UUIDs can be found in these two files (they get updated every Mac OS X update, and thus break plugins each time):