A signature cannot be an assertion of any particular anti-spam policy.
I agree with that, but will go further. A signature cannot be an
assertion of goodness. A signature can say things like "Alice wrote
this message" or it can say "Bob resent this message to Carol" or it
can say "MTA foo.example.com forwarded this message to Dave".
A signature can even say "Ed attests that this message contains only
content about topic category Y" or "Fred attests that this message
relates to your transaction Z".
What a signature cannot meaningfully say is "this is something you
should read", because every signer will use his own criteria for that,
and the signer will bias those criteria in his favor. There can be
more than one kind of signature, but the meaning of each kind of
signature needs to be standardized for the signatures to be useful as
any more than an indicator of "I once saw this hash of this set of bits".
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