Then we're screwed, since it'll be impossible to do anything at all
that makes it more likely to get your mail delivered.
That's still blurring the line between a verifier and an assessor. We can only
make educated guesses about the latter, but we have substantial influence on
the former.
Subject: isn't a mandatory header per RFC5322, if you're using that as a
specific example.
Remember that a header doesn't have to be present to be signed. It'd be an
egregious spamability hole to let people add signatures and replay without
breaking the signature.
Oh, I haven't forgotten that. It goes hand-in-hand with the "display header"
idea.
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