On 22.08.23 18:43, Peter H via Postfix-users wrote:
When my mailserver talks to other MTA, it certainly will issue a HELO
command.
Saying the hostname after HELO is: mail.host.com, which points to an IP.
But this IP's PTR doesn't point back to the hostname above.
That's saying, the IP does have a PTR like web.host.com, and
web.host.com points to that IP correctly.
This mapping does not matter, so far all SMTP RFCs have specified that
hostname specified in HELO needs NOT to match the result of client IP's
hostname lookup.
However, there are hosts that refuse mail from you if your IP does not
resolve, or it resolves to hostname that does not resolve back to that IP.
It's called FcrDNS.
But my mailserver doesn't use web.host.com as HELO hostname, instead
it's using mail.host.com which has correct A RR, though that IP
address doesn't resolve back to mail.host.com.
This does not matter, if you provide hostname, servers check if it resolves
to an IP address, nobody checks reverse DNS of that address.
Does this have an adverse effect on sending behavior?
No. You only need to provide FQDN that has address record (and not a CNAME).
There are servers that refuse mail from hosts whose reverse DNS resolves to
generic name (ip-a.b.c.d) or dynamic address (or, maybe, if your HELO
provides such name).
--
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