Hello,
I recently installed Mailcow and I’m encountering some difficulties with receiving emails. I’m not sure if you’ll be able to help me. I apologize in advance for my poor English.
I noticed that I’m not receiving certain emails, and after investigating a bit, I realized that the issue is happening at the Postfix level.

08/04/2025 22:05:43 info NOQUEUE: reject: RCPT from unknown[91.211.164.20]: 450 4.1.8 <noreply@ldlc.com>: Sender address rejected: Domain not found; from=<noreply@ldlc.com> to=<exemple@exemple.com> proto=ESMTP helo=<mail.groupe-ldlc.com>

My first instinct was to run a “dig” from the different containers.

Posfix:

dig ldlc.com => KO

root@7325df837d6f:/etc/postfix# dig ldlc.com

; <<>> DiG 9.18.33-1_deb12u2-Debian <<>> ldlc.com
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL, id: 26655
;; flags: qr rd; QUERY: 1, ANSWER: 0, AUTHORITY: 0, ADDITIONAL: 0
;; WARNING: recursion requested but not available

;; QUESTION SECTION:
;ldlc.com. IN A

;; Query time: 4003 msec
;; SERVER: 127.0.0.11#53(127.0.0.11) (UDP)
;; WHEN: Tue Apr 08 22:39:00 CEST 2025
;; MSG SIZE rcvd: 26

dig google.com => OK

root@7325df837d6f:/etc/postfix# dig google.fr

; <<>> DiG 9.18.33-1~deb12u2-Debian <<>> google.fr
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 16527
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1

;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 1232
;; QUESTION SECTION:
;google.fr.                     IN      A

;; ANSWER SECTION:
google.fr.              300     IN      A       142.250.179.99

;; Query time: 395 msec
;; SERVER: 127.0.0.11#53(127.0.0.11) (UDP)
;; WHEN: Tue Apr 08 22:39:40 CEST 2025
;; MSG SIZE  rcvd: 54

Dovecot

dig ldlc.com => KO
dig google.com => OK

Unbound

dig ldlc.com => OK
dig google.com => OK

On the machine that hosts all the containers.

dig ldlc.com => OK
dig google.com => OK

My server has both an IPv4 and an IPv6 address.

If you need more information, don’t hesitate to ask.
Thank you for your help. 😉