the question asked was "what are the advantages/disadvantages of each
approach?" the answer that came back only sheds light on what the majority of
vendors implement (or, more precisely, don't implement).
i would like some clarification on this as well, including, but not limited to,
whether this is relevant to the following circumstance:
person A receives a message & attachment signed by person B (perhaps from a
previous email) and forwards this along to person C. of course, person A wants
to put his own signature on the entire email, so person C is sure where the
forwarding message came from.
in general, how does a user forward a message such that the recipient can prove
that (a) the original message came from the purported original sender, and (b)
that this forwarded message and any additional text/attachments are from the
purported forwarder/intermediary.