Postfix: host or domain name not found

Postfix: Host or domain name not found. Name service error for name=… type=MX

Postfix could not look up the recipient domain in DNS. Either your server's resolver does not work (often a chrooted Postfix without resolv.conf), or the domain itself has a problem.

daniele@mikweb: ~/errors — less postfix-host-or-domain-name-not-found-name-service-error.md

What the message looks like

postfix/smtp[2314]: 4F1A2B3C4D: to=<kunde@example.net>, relay=none, delay=0.5, dsn=4.4.3, status=deferred (Host or domain name not found. Name service error for name=example.net type=MX: Host not found, try again)

Common causes

The DNS resolver is unreachable or misconfigured: /etc/resolv.conf, systemd-resolved or a blocked port 53.
Postfix runs in a chroot and does not find a copy of resolv.conf there.
The recipient domain has a typo, no MX or A record, or its name servers fail.
A local firewall blocks outgoing DNS queries.
A short-term DNS outage: “try again” means the error is temporary.

Check

Does the lookup work from the shell?
dig MX example.net +short
Which resolver is configured?
cat /etc/resolv.conf
Does the Postfix chroot have its own copy?
ls -l /var/spool/postfix/etc/resolv.conf
Related Postfix settings
postconf -n | grep -E 'disable_dns_lookups|smtp_host_lookup|myhostname'

Fix

Fix the resolver, or install a local caching one such as unbound, so DNS no longer fails.
Refresh the chroot copy and restart: cp /etc/resolv.conf /var/spool/postfix/etc/resolv.conf && systemctl restart postfix.
Check the recipient address for typos and the domain with dig MX.
Allow outgoing UDP and TCP port 53 in the firewall.
After the fix, retry the queue: postqueue -f.

FAQ

Is “try again” a permanent error?

No. It is a temporary failure (4.x.x). Postfix keeps the message and tries again for several days.

How do I check my own domain's mail DNS?

Use the free mail DNS checker for MX, SPF, DKIM, DMARC and PTR.