From my perspective having a message have a valid signature with one
implementation and having a broken signature with another is an
incompatibility. I don't think that's speculation. ...
No, it merely reflects a difference of opinion by the sites concerned as
to what changes it will tolerate in a message before it recommends to its
clients that the message should be dropped. It is not the job of our
standard to dictate local policy issues at that level of detail.
I agree that we are not dictating local policy. But I really think that
it's our job to dictate the definition of what the signature validation
algorithm is. As I've said before, everyone remains free to do whatever
they want with messages whether or not the signature verifies, including
applying various heuristics to develop opinions about unsigned messages.
Perhaps some people are confusing verification and presentation.
Verification: it is critical that all DKIM verifiers agree on what
is a valid DKIM signature, without falling back on heuristics, such
as heuristics to repair messages.
Presentation: after the valid/invalid decision is irevocably made,
it is up to application/policy to decide how/if things will be
presented to users. Heuristics of various sorts can be useful in
this domain, such as message repair, known signer associations,
etc., but those heuristics must not determine the validity of the
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