Add a new paragraph at the end of section 1.1:
A DKIM signature is intended to be verifiable for about the length of
time of that it would take to transport the message in SMTP; this is
almost always less than two weeks. The signature is not intended to
be used for archival purposes, and verification of the signature after
this period is not an intended or desired feature of the DKIM protocol.
This text doesn't seem to be able to make up its mind if it's informational
or normative. It needs to be normative. Which is why x= is better since
we can recommend something squishy like around 2 weeks, but be able
change it in the future if needed.
Mike
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