A reasonable verifier can completely ignore x= and still get the right
result in all non-silly cases, which tells me that x= should go.
[...]
PS: This is the same reason that l= should go.
I think "l=" and "x=" both put more information into the hands of the
verifiers. It's true they can ignore such information, but they can also elect
to observe things like "Hmmm, the verifier didn't intend for this to verify
beyond time T. Maybe I should treat it differently somehow."
Same goes for "l=". And the opposite goes for "l=" as well; a verifier could
decide it will not accept mail that has had more than a certain volume, or
percentage, of additional text beyond what the signature covers. I know of one
such implementation already.
Just because one verifier (or even many) could decide to ignore "x=", doesn't
mean all of them will want to do so.
I'm not sure I agree with the rhetoric that DKIM's base spec should include
absolutely nothing other than the bare essentials. I'm a fan of providing more
information whenever possible.
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