Hello Wietse, SpamAssassin recomends inserting fake (lowest and highest) MX records https://cwiki.apache.org/confluence/display/spamassassin/OtherTricks to reduce Spam. E.g. MX aegee.org resolves to 90 mxf-2.aegee.org. / 10 mail.aegee.org. / 1 mxf-1.aegee.org. and on mxf-1,2.aegee.org there is no SMTP server.
That said, my understandig is: A mail has 3 recipients and is sent to a domain with fake MX records deployed, but otherwise has 50 distinct accepting IP addresses behind the MX records. The smtp server accepts one recipient per transaction and 451 defers the other recipients. Postfix picks up to 5 (smtp_mx_address_limit) distinct IP addresses from DNS. Postfix will connect first to the fake MX host. Then postfix connects to two other hosts and each host consumes one recipient. Then postfix gives up and retries delivery to the last recipient later. Why doesn’t postfix handle the 4.5.3 status code in a special way? As long as per iteration the number of recipients is reduced, keep retrying without giving up. Regards Дилян On Mon, 2019-07-29 at 11:18 -0400, Wietse Venema wrote: > Wietse: > > When a recipient fails with 4xx, Postfix does not give up immediately, > > BUT WILL TRY THE REMAINING MX HOSTS, up to the limits specified > > with $smtp_mx_address_limit and $smtp_mx_session_limit. > > OP: > > My reading of your answer is, that by default, if some recipients > > are accepted and others 451/452 rejected, postfix closes the SMTP > > session, reopens a new SMTP session to the same host and tries > > again to that host (smtp_mx_session_limit > > default 2). > > No, see CAPITALIZED text above. Postfix does this for all 4XX > recipient replies, not just 45X. > > Wietse
