Mutation of header fields can be dealt with if the signer is able to
save the contents of the fields it signs, and those saved versions
are what is used during verification. Verifiers can then restore
the save versions if verification passes.
You could write software that did that, but I have never seen a
plausible scenario where it'd be useful.
How many mailing lists do you subscribe to where you need to do spam
filtering on the mail that comes through the list? Me neither. (I do
bozo filtering, but the bozos are neither spammers nor forgers, just
bozos, and I filter on their From: lines.)
If bad guys try to turn the clock back to 1995 and use mailing lists
as spam amplifiers, the solution will be the same it was ten years
ago, to fix the list software to keep the bad guys out, not to do
filtering at the recipient end.
R's,
John
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