On Mon, Apr 14, 2025 at 11:23:49AM +0200, Benoit Panizzon via Postfix-devel
wrote:
> As smtpd milter we use:
>
> MIMEDefang (using spamassassin / clamav)
> milter-greylist
> opendmarc
>
> Every now and then, an email to multiple recipients is constantly being
> rejected by Postfix with:
>
> milter-reject: RCPT from (sending server) 451 4.7.1 Service unavailable - try
> again later;
"milter-greylist" would be the main suspect for doing something
non-trivial at the "RCPT" stage. The others can't do much without the
message content. But isn't this just a normal "greylist" action??? If
there's no logging about milter timeouts or failures, no other
explanation comes to mind.
> First handful of recipients show up in the MIMEDefang Log. Then there
> is a 30 seconds gap, then Postfix fails all remaining recipients and
> the complete email with 4.7.1 If one email triggers that issue, it is
> triggered every time, on all of our 3 MX with identical milter config.
But, it is also possible that spamassassin (via MIMEDefang) is
configured to do non-trivial sender or recipient DNS lookups, and these
may be timing out. I think we can rule out opendmarc.
> I start to suspect, that one of the other milter fails. Is there a way
> to log the milter commands sent from postfix to each milter from withing
> postfix to find out which milter is 'timing out' on RCPT?
What do ordinary greylisting actions look like? Is the milter
configured to tarpit the sender? Otherwise, your culprit is
SA, likely trying and failing DNS lookups. Perhaps SPF?
This is a postfix-users question, please respond there.
--
Viktor.
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