Well, let's get back to first princples. RFC 2822 3.6.2:
"The "Sender:" field specifies the mailbox of the agent
responsible for the actual transmission of the message."
Heretically rejecting the RFC 2822 restriction to one Sender header
field appears to put things back in order and opens up new grounds.
There could be one Sender for the secretary (mailing on behalf of...),
one by a googlemail.com remailer, one by a final mailing list
(each one prepended to a header), and each hop may sign whatever
fields it desires, ragardless of what might next hops be.
Mark
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