Error while trying to attach a signature

Hi! I hope anyone can give a clue about this…

This person is trying to insert his signature in an email (he doesn’t have “Automatically append a signature to the bottom of my outgoing mail messages” option checked because is annoying to him), so when he tries to attach by doing TOOLS → Insert signature, he gets

“MESSAGE WITH SUBJECT LINE: (subject) APPEARS TO HAVE COME FROM AN INTERNAL USER AND CANNOT BE FORWARDED TO THE ADMIN TEAM FOR EVALUATION”

I appreciate any clue you can provide.

Greeetings from Argentina =).