I've been looking at what do different mail redirection systems (mail lists
in particular) do with emails and discovered that some (especially newer) mail lists treat S/MIME and PGP/MIME signed email different then other email they process (not surprising and this is obviously a good thing).
In particular what I found interesting cases when mail list saw multipart/signed as primary Content-Type in the email header, it made that into separate multipart/signed mime part and made the email itself multipart/mixed consisting of this multipart/signed part as first one and then text/plain with mail list signature.
So in this case email list not only added text at the end but in the middle and changed content-type and I imagine this would be a problem
for IIM. This case is also why I said when I first saw DK draft about
year ago, that for mime messages, signature should be linked to mime part(s) rather then be for entire message.