One way to disagree is about IP address family.
If I remember correctly, nothing (else) in 5321/2 requires a v6-only host
to understand anything about v4. Running a v6-only host is stupid at this
point, perhaps slightly less stupid in five years, but even so I don't
think the next RFCs should feature an IPv4 requirement for address
literals, and nothing for the main parts of the protocols.
Assuming we keep address literals as part of SMTP (which I think would
be a mistake) rather than submission (where I don't care), I think the
as-if rule applies.
If a host doesn't get v4 connections, it doesn't need to recognize v4
address literals. Similarly, if it doesn't get v6 connections it
doesn't need to recognize v6 literals. Standards are about
interoperation, and cases that never happen don't matter for interop.
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