A more interesting case to consider is acm.org style forwarders,
where the forwarder is, in many ways, the final destination, and where
the address at the forwarder is "owned" by the final recipient, and
where they will likely ask for transactional mail of the sort that
senders might consider discardable be sent.
Our working hypothesis has been that most forwarders of that sort won't
break the signature, so it's not a problem.
The flame wars erupt when you ask who, if anyone, should sign mail from
domains like acm.org that are sent from a thousand random networks around
the world where ACM members happen to get their mail.
R's,
uucp(_at_)acm(_dot_)org
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