Recently I have been playing with the coolness of Digitally Signed Emails, and I noticed something odd. Emails sent within my domain worked fine however when my messages left the domain, winging their way to a better place, they arrived with no body. Just the Disclaimer and a very curious .DAT file.
The problem was that my Exchange server appends a disclaimer to the any email leaving the domain by way of a transport rule. As the whole point of digitally signing the message is to prove that the message has not been altered in transit, Exchange cannot add the disclaimer. So, instead, it replaces the message with the new message (containing just the disclaimer) and then adds the actual signed message as an attachment. However not all email clients understand what a DAT file is, and therefore causes some confusion at the other end.
You can read more about it here: http://technet.microsoft.com/en-us/library/dd535377(EXCHG.80).aspx
My only solution at this time, to be able to send the signed messages, is to add an exclusion to the rule for my account. Not perfect... or ideal, but it has proved where my complication is.