Therefore, a verifier SHOULD NOT validate a message that is not
compliant with [RFC5322, RFC2045 and RFC2047] specifications.
IMHO, it is somewhat vague. That SHOULD-NOT could be "promoted" to a
MUST-NOT for a finite number of specific features --to be explicitly
listed for readers' convenience.
I'm pretty sure we already had this argument, and SHOULD NOT was the
rough consensus.
R's,
John
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