On Sun, 15 Dec 2019 16:18:11 -0500, Viktor Dukhovni said:
As John Klensin observed, I did not suggest (nor did anyone else in this
that address literals should be prohibited. They are fine to use in private
networks, and "mostly" work even on the public Internet, but (hearsay) over
years I've seen repeated posts on e.g. the Postfix users list, where sysops
using regexp HELO name filters that would block address literals.
I admit not knowing how to express in RFC language the concept that the MTA
code MUST (2119 sense) allow address literals, while email admins be advised
that accepting them on the open Internet is, as a practical matter, somewhere
between MAY NOT and SHOULD NOT (but not rising to MUST NOT). In three decades
of dealing with RFCs, I can't recall a prior instance where we've ended up
mandating that an implementation support something that's usually a bad
idea except in specialized corner cases...
That's why I let John Klensin write them. :)
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