On Tue, 20 Jun 2023, Mik J wrote:
I have this filter configured
filter check_rdns phase connect match !rdns disconnect "550 no rDNS is so
80s"
Someone else answered your real question, so please excuse my rant. I
note that rDNS is still not part of the SMTP RFCs. HELO is what the
standard specifies, and checking that HELO resolves to the connect IP
accomplishes the same as rDNS.
While IPv6 has improved this, requiring rDNS for IPv4 causes problems
for the little guy. You have to have at least a Class C block.
(Some ISPs support CNAME based rDNS delegation for IPv4.)
Even though IPv6 makes rDNS much easier for the little guy, too many
IPv6 ISPs on the backbone simply do not support it. (he.net does fully
support it.)
Back in the 80s, there were a lot of clueless email admins (hey, at
least they weren't using gmail) that couldn't figure out what
"hostname of the SMTP client" means for HELO. So rejecting on invalid
HELO got a lot of real business emails with clueless admins. The rDNS
hack was a substitute when HELO was invalid, as a class C was typical
for even a small business back then. No more.
