Mail loops back to myself for just one sender

drBeam

Hi community,

i have some struggle with an incompetent insurance company (DA-Direkt) which is for some reason sending mails to my backup mailserver and for some reason that i dont understand my backup mailserver does not know how to send this to my primary mailserver. For now the mail is stuck in the Queue on my backup server and it says “mail loops back to myself” while the primary server is clearly working and everything else gets delivered, even my test mail loop which sends a mail to my backup mx to forward it to my primary mx.

What is going on there and how can i fix this? It would be enough to pull out the attachment out of the queue but i don’t know how to do this.

Thanks & best regards
drbeam

esackbauer

This why I do not like backup mailservers… for the insurance company their mail is now delivered. This can have implications in case of a lawsuit. Usually backup mailservers are not worth the hassle.
I think if both mail servers are in your MX record as sender you are free to chose any of them. Maybe your primary does not fulfill some criterias (latency, routing issue).

What exactly is the “backup mailserver” and what is it running? Any forwarding or sieve rules active for that mailbox?

drBeam

esackbauer The Backup Mailserver is also a mailcow setup. All Domains are set as “relay all”. Pretty much as configured here: https://autoize.com/backup-mx-configuration-for-mailcow/

MX Records are also correct. I just wonder why the mails that are sent to the backup mx are not delivered to the primary. Something is wrong with the messages from them.

I have a check_mk check configured to test the mail loop every few minutes. It sends a message to the backup server. The backup server sends it to the primary where the check_mk checks if the mail is delivered. This is working absolutely fine.

drBeam

Update: as always, it seems to be caused by DNS 🙂

My primary server rejects mails with the domain da-direkt.de. It can’t resolve it. Everything else works and i can see the DNS queries in my DNS-Servers. From every other machine it works, even from the mailcow postfix container if i specify the DNS servers manually when running nslookup.

drBeam

Fixed it!

I configured my DNS-Server according to this doc: https://docs.mailcow.email/manual-guides/Unbound/u_e-unbound-fwd/

The final hint was this line: 450 4.1.8 kundenservice@da-direkt.de: Sender address rejected: Domain not found (in reply to RCPT TO command).
I tested nslookup from the postfix container and everything was resolveable except da-direkt.de. No idea why it didnt work but it works now.