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 9.18.33-1deb12u2-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. 😉

  • DocFraggle
    Thank you for your response. Indeed, the issue was coming from the “Edge Network Firewall” of my hosting provider, OVH. I hadn’t really checked that point because most DNS resolutions were working fine or, in some cases, only partially. I also adjusted some long response times on my SMTP along the way.

    Run

    dig +trace ldlc.com

    to see where it’s failing

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      DocFraggle

      root@7325df837d6f:/# dig +trace ldlc.com

; <<>> DiG 9.18.33-1~deb12u2-Debian <<>> +trace ldlc.com
;; global options: +cmd
.			44195	IN	NS	l.root-servers.net.
.			44195	IN	NS	c.root-servers.net.
.			44195	IN	NS	a.root-servers.net.
.			44195	IN	NS	k.root-servers.net.
.			44195	IN	NS	j.root-servers.net.
.			44195	IN	NS	d.root-servers.net.
.			44195	IN	NS	i.root-servers.net.
.			44195	IN	NS	e.root-servers.net.
.			44195	IN	NS	f.root-servers.net.
.			44195	IN	NS	g.root-servers.net.
.			44195	IN	NS	h.root-servers.net.
.			44195	IN	NS	b.root-servers.net.
.			44195	IN	NS	m.root-servers.net.
.			44195	IN	RRSIG	NS 8 0 518400 20250421170000 20250408160000 53148 . HEfVigBsoJQoHTY26YsMjEfiu4GXYus0kZOPVhf/faffwq6OZvYZ283y hjIt5alPBCHyCgXS6FI5GuDs0CP/NpYY1HpL5MNf4Kuyi0i1HWIPGTZB 5vKijMWtBPcRnXFQOlgHwsrvzZqMs4LW5yDt6PSGG/8r4IYVceSTVNH3 Lbbg25y7Q5qp8KgnS3JjACk1u+kTHwPl2M5RZj9n7E3HUuxfa5sN08rc bsXVs7wDk1JCgNTaRGNmhhYhEHqP0tZ0A2pMKjZtxdCHDCRgnZYrw1Nk LzMEaY2alikCfFjpaTIuL1f/i6gxwUZjs2YwyNX/BsDrG9tRCA/KJkRl SHHoNA==
;; Received 525 bytes from 127.0.0.11#53(127.0.0.11) in 0 ms

com.			172800	IN	NS	j.gtld-servers.net.
com.			172800	IN	NS	c.gtld-servers.net.
com.			172800	IN	NS	e.gtld-servers.net.
com.			172800	IN	NS	k.gtld-servers.net.
com.			172800	IN	NS	i.gtld-servers.net.
com.			172800	IN	NS	g.gtld-servers.net.
com.			172800	IN	NS	m.gtld-servers.net.
com.			172800	IN	NS	b.gtld-servers.net.
com.			172800	IN	NS	d.gtld-servers.net.
com.			172800	IN	NS	h.gtld-servers.net.
com.			172800	IN	NS	l.gtld-servers.net.
com.			172800	IN	NS	f.gtld-servers.net.
com.			172800	IN	NS	a.gtld-servers.net.
com.			86400	IN	DS	19718 13 2 8ACBB0CD28F41250A80A491389424D341522D946B0DA0C0291F2D3D7 71D7805A
com.			86400	IN	RRSIG	DS 8 1 86400 20250421200000 20250408190000 53148 . ddXeXhrcxQ5H/1tkqEg25/Xb4QTBNdqf56t0EXP/hFwh/+MwVnMD4ORz 0dWiq1CL2mzKDIXCFEfzjzW90STjKhfSj1hQhdMqvMZV0Pny7zIJ1b5x vJPEPHf8Awb6/dgjszEz57UCxE57oF4FP9Ru0/f8oTJlqD911ay4n7y5 Q9Upnj5T/3QDQmuWQ1G5prlZPSJrVbPyfbxwry1u3nbixr/V7+T6sCh6 59c97om+fFW+k6Vgy1V/oE81hAv2quoVplZj8VlNpbAriuJF9ItmQHEW fkNX0giXiAugkE66QUHwD5jWgnW4EsJoAvGUyCI8ruRp2szu8HKpos1G aifaoQ==
;; Received 1196 bytes from 2001:500:2::c#53(c.root-servers.net) in 7 ms

ldlc.com.		172800	IN	NS	ns10.groupe-ldlc.com.
ldlc.com.		172800	IN	NS	ns20.groupe-ldlc.com.
ldlc.com.		172800	IN	NS	ns30.groupe-ldlc.com.
CK0POJMG874LJREF7EFN8430QVIT8BSM.com. 900 IN NSEC3 1 1 0 - CK0Q3UDG8CEKKAE7RUKPGCT1DVSSH8LL NS SOA RRSIG DNSKEY NSEC3PARAM
CK0POJMG874LJREF7EFN8430QVIT8BSM.com. 900 IN RRSIG NSEC3 13 2 900 20250413021040 20250406010040 23202 com. q2m1iXdbJn0UrtzQuUdkWW3rTB9+0zhmZADiPGEhxx6T+jilxd4HRJib f0mv41QUxTzRyNg4FaWdgXNQfQcCuQ==
A0JS98UQ20CQ5DEOKDDSI3MOQCF5O39C.com. 900 IN NSEC3 1 1 0 - A0JSD7GUHMIETL081PKSOATA41P6R8DR NS DS RRSIG
A0JS98UQ20CQ5DEOKDDSI3MOQCF5O39C.com. 900 IN RRSIG NSEC3 13 2 900 20250413024610 20250406013610 23202 com. VcJCWwEXpm0vAPxRXgWwfVKaJjtMUITJckR2UibHni0nGTBg6Vyz8DMa HmBwaAsTM210p5qwCEOP4fJysUCh9A==
couldn't get address for 'ns10.groupe-ldlc.com': failure
couldn't get address for 'ns20.groupe-ldlc.com': failure
couldn't get address for 'ns30.groupe-ldlc.com': failure
dig: couldn't get address for 'ns10.groupe-ldlc.com': no more

          Gamodeur

          couldn't get address for 'ns10.groupe-ldlc.com': failure
couldn't get address for 'ns20.groupe-ldlc.com': failure
couldn't get address for 'ns30.groupe-ldlc.com': failure
dig: couldn't get address for 'ns10.groupe-ldlc.com': no more

          That’s strange… do you have any kind of external firewall or a separate firewall running on the host system which may block packets? selinux maybe?

              DocFraggle
              Thank you for your response. Indeed, the issue was coming from the “Edge Network Firewall” of my hosting provider, OVH. I hadn’t really checked that point because most DNS resolutions were working fine or, in some cases, only partially. I also adjusted some long response times on my SMTP along the way.

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