And while clients SHOULD use a DNS name in HELO/EHLO if they have one,
not all SMTP clients can be expected to have a DNS name. So servers
SHOULD NOT (maybe MUST NOT) reject mail based on the mere presence or
absence of an IP address literal in HELO/EHLO. It would be insane for
a standard to make a recommendation that degrades the reliability of the
service it provides.
I am reasonably sure that if we compare the number of MTAs sending
legit mail with numeric HELO to the number of spambots doing so, mail
reliability would be greatly improved by completely forbidding them.
With respect to all of the low performance IoT devices mailing out
status reports, that's submission, not SMTP. You point them at a
submission server that knows what network(s) the clients are on,
cleans up the messages, perhaps does some sanity checking so a rogue
lightbulb can't mailbomb people at qq.com, and forwards on the
messages as an MTA. A Raspberry Pi is overkill for this.
We definitely need to make it clear in 5321bis that submission and
SMTP are different, and devices that can't do everything that a modern
SMTP client needs to do have to use a submission relay. It's store and
forward. The mail gets through anyway.
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