From: John L [mailto:johnl(_at_)iecc(_dot_)com]
Its not enforceable as it is at the option of the receiver and so
cannot be a MUST NOT, it could be a SHOULD NOT.
Seems to me this is a semantic niggle. People can implement
whatever variant form of whatever spec they want to, but if
they want to interoperate and implement the signature
validation that DKIM specifies, that have to use the current
values of the headers.
Semantics is from the Greek, it means 'meaning'.
There is no part of writing a standard that is not a semantic exercise.
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