Validating mail syntax belongs in the specification for the mail
components and DKIM work belongs in the DKIM components.
Yes and no.
The problem for me is that in most situations only outgoing or relaying
MSAs do format validation, and on incoming mail, some 5322 violations are
considerably worse than others. I've never seen a legit message with two
From or Subject headers, I've gotten plenty with a message-id with an
extra at sign or a MIME-Type but no MIME-Version. I can somewhat see
inisisting on validating all of that before signing, to encourage people
to clean up their sending software, but if you're that strict on
verification, you're going to lose signatures on valid but sloppy mail.
That's why, layer violation or no, I think it's important to distinguish
between format errors that are likely to lead to misleading rendering in
existing MUAs, and the much larger class that may produce nonsense but
won't produce lies.
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