On Thu, Sep 11, 2025 at 11:40:47PM +0000, Rob Hoo via Postfix-users wrote:
> I know that a sender host should have correct dns records for either A
> or PTR.
The IP address of an SMTP client needs to have an associated PTR record
(possibly after CNAME expansion of the re.ve.rse.ip.in-addra.arpa query
name), whose value (a new DNS name) in turn resolves back to a list of
IP addresses that includes the original IP address, example:
; Simple reverse mapping
;
1.2.0.192.in-addr.arpa. IN PTR mta-name.example.
;
; OR else CNAME indirected reverse mapping
;
1.2.0.192.in-addr.arpa. IN CNAME foo.example.
foo.example. IN CNAME bar.example.
bar.example. IN PTR mta-name.example.
; Forward mapping of the PTR name:
;
mta-name.example. IN A 192.0.2.1
mta-name.example. IN A 192.0.2.2
...
The resulting "mta-name.example" is said to be a forward-consistent
reverse DNS name (FCrDNS).
> for HELO hostname, does it require the same settings (both A and PTR are
> set, and matching).
Most receiving systems don't require that the HELO name (forward)
resolve to the IP address of the SMTP client system, just being
a valid DNS name is generally sufficient, but a minority of
receiving systems may require such a match. So the below should
be OK for an SMTP client with an external IP address of 192.0.2.1:
helo.name.example. IN A 192.0.2.1
; And as above:
1.2.0.192.in-addr.arpa. IN PTR mta-name.example.
mta-name.example. IN A 192.0.2.1
mta-name.example. IN A 192.0.2.2
No PTR records are involved, the HELO name either resolves to the
client's address or it does not. Of course the simplest configuration
is one in which the HELO name is the same as the FCrDNS name obtained
via PTR lookups.
--
Viktor. 🇺🇦 Слава Україні!
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