John C Klensin writes:
regional registries. Independent of clients who are part of
clouds and business arrangements large enough to push ISPs
around, many ISPs refuse to create reverse mapping records at
all  or will not create ones that that match client
perceptions of their names rather than server-location based
names following the ITU model /
 As handy examples pulled from this thread, 18.104.22.168 and
22.214.171.124 apparently have no reverse mapping records at all.
;; ANSWER SECTION:
126.96.36.199.in-addr.arpa. 4156 IN PTR mailx.courier-mta.com.
;; AUTHORITY SECTION:
166.68.in-addr.arpa. 71418 IN NS ns3.gtt.net.
166.68.in-addr.arpa. 71418 IN NS ns2.gtt.net.
166.68.in-addr.arpa. 71418 IN NS ns1.gtt.net.
The somewhat cumbersome https://mxtoolbox.com/ReverseLookup.aspx verified
that it's not just the little bubble of the Intertubes that I live in can
see this reverse mapping.
Anyway, the only point I made is that I believe that the MUST NOT, when it
comes to EHLO/HELO validation, is frequently ignored in practice. Those who
are of the opinion that validation is bad practice, nobody will force them
to do it. And those who do believe that it filters out a bunch of crap will
do it; they will ignore the MUST NOT. That's it.
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