On 4/17/2023 9:35 PM, tom--- via Postfix-users wrote:
When my main postfix rejected a message due to
reject_unknown_client_hostname:
Apr 18 10:27:12 mail postfix/smtpd[129429]: NOQUEUE: reject: RCPT
from unknown[194.33.39.17]: 450 4.7.25 Client host rejected: cannot
find your hostname, [194.33.39.17]; from=<x...@a.com> to=<y...@b.com>
proto=ESMTP helo=<mx.a.com>
I found this message was re-delivered by peer MTA to my secondary MX
quickly.
So my question is, smtp code 450 will cause the sender to retry
delivery to secondary MX?
After a 450 "defer" response, the sender is free to retry any time
to any MX - immediately retrying a secondary MX is a reasonable action.
Also see:
http://www.postfix.org/postconf.5.html#unknown_client_reject_code
and consider changing it to 550 to permanently reject unwanted mail,
rather than deferring it.
This is one reason that all MX servers should have the same spam
controls and valid recipient lists. and an excuse to get rid of
largely unnecessary secondary MX servers.
Note that reject_unknown_client_hostname is a very strict test that
is likely to reject legit mail. Consider using
reject_unknown_reverse_client_hostname instead.
-- Noel Jones
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