I think the trouble with this attachment is that the whole e-mail is encrypted "in clear" (anybody can decrypt) to save space when you send the e-mail (SSL/TLS includes compression).
It's not encrypted, it's encoded in a form (base 64) that is unlikely to mangled by "helpful" mail relays, and thus ensures that the signature is likely to not fail because of "friendly fire."
Let's be accurate in our comments.
Steve