Yes, we had this same problem with XMLDsig. We can sign/encrypt the data in the payload but we will miss the MIME headers. A better solution is to encrypt the entire message in one shot, thus eliminating these holes. It will also be more efficient. Where we got into trouble was when we pulled the values (AckRequested, SyncReply) out of the Via element and made them top level elements. We could have just excluded the Via element and thus any changes within that element would not have invalidated the signature. Studying the MS document would be a good idea. I just read this document and I didn't see it address either of our concerns, namely the problems caused by actor=next and the issue of signing attachments. Attachments are mentioned in passing (section 3.1) but not actually addressed. I will read it again. David.