As I read the later case, the only signature present (C's) is not one that is
included in A's SSP. In this case we have a message with a signature that is
outside the scope what A has said is authorized (or not included in A's
authoritative list). If A is a high profile phishing target and signs all of
it's mail, then it would be useful (I think) for receivers to recognize that
the message has been signed by someone other than who A said it would.
Why do you want to prevent people from forwarding genuine, unmodified
messages? That's a feature, not a bug.
If ebay sends a message with a valid ebay signature, how can any chain
of forwarding and added signatures change the fact that it's a real
ebay message? Let's assume that ebay has enough sense to sign its
MIME headers and not to use l=, so the message that's delivered is the
same one that was sent.
R's,
John
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