I haven't used DCC, but it looks like an interesting anti-spam toolkit: https://www.dcc-servers.net/dcc/ https://www.dcc-servers.net/dcc/ It appears to suppor

I haven't used DCC, but it looks like an interesting anti-spam toolkit: https://www.dcc-servers.net/dcc/ https://www.dcc-servers.net/dcc/ It appears to support "weak" IP matching: > All or part of the IP address of the SMTP client can be optionally ignored by DCC clients as far as the greylist triple is concerned. This feature may be useful for legitimate mail systems that shuffle messages among SMTP clients between retransmissions. See the dccm and dccd man pages. https://www.dcc-servers.net/dcc/greylist.shtml https://www.dcc-servers.net/dcc/greylist.shtml It doesn't quite sound like it does a job of 100% "same email, same sender, different mx in same domain" -but I suspect it works well enough in practice? > Usually the DCC greylist system requires that an almost identical copy of the message be retransmitted during the embargo. If weak-body is present, any message with the same triple of sender IP address, sender mail address, and target mail address ends the em-bargo, even if the body of the message differs. > If weak-IP is present, all mail from an SMTP client at an IP address is accept after any message from the same IP address has been ac-cepted. https://www.dcc-servers.net/dcc/dcc-tree/dccd.html#OPTION-G https://www.dcc-servers.net/dcc/dcc-tree/dccd.html#OPTION-G I can't recall what I used for greylisting last -possibly greylistd. Anyway, the smart play these days might be to whitelist/greylist via SPF - I'm not sure if spammers (of the variant caught by greylists) generally have SPF? See: https://poolp.org/posts/2019-12-01/spf-aware-greylisting-and-filter-greylist/ https://poolp.org/posts/2019-12-01/spf-aware-greylisting-and... https://github.com/poolpOrg/filter-greylist https://github.com/poolpOrg/filter-greylist Ed: although if service providers like mailgun simply ignore rfc and only "sometimes" retries... wwell that's a problem.