Postfix warning: hostname … does not resolve to address

What the message looks like

warning: hostname mail.example.net does not resolve to address 203.0.113.25: Name or service not known

Common causes

• The sender's PTR points to a name without a matching A or AAAA record.
• Typical for spam bots and residential lines – then the warning is expected.
• Your own server: PTR set at the hoster, but the A record is missing or points elsewhere.
• IPv6 connection while only an IPv4 reverse entry exists.
• The local resolver returns stale answers or none at all.

Check

1. Look up the PTR of the connecting IP
   dig -x 203.0.113.25 +shortCopy
2. Check that the name points back to the same IP
   dig +short A mail.example.net; dig +short AAAA mail.example.netCopy
3. Count the most frequent IPs in the log
   grep 'does not resolve to address' /var/log/mail.log | awk '{print $NF}' | sort | uniq -c | sort -rn | headCopy
4. Check whether strict hostname checks are enabled
   postconf smtpd_client_restrictions smtpd_recipient_restrictions | grep -o 'reject_unknown[a-z_]*'Copy

Fix

• If it concerns foreign IPs, the warning is normal and can be ignored.
• If it is your own server: set the PTR at your provider and create an A or AAAA record with exactly that IP.
• Prefer reject_unknown_reverse_client_hostname over reject_unknown_client_hostname – the strict one also hits legitimate servers.
• For IPv6 sending set a PTR for the IPv6 address, or send over IPv4 only (smtp_address_preference = ipv4).

FAQ

Is mail rejected because of this warning?

No, the warning alone rejects nothing. Mail is only rejected if you set reject_unknown_client_hostname – then with 450 4.7.25.

Is this a problem with Gmail or Microsoft?

For your own outgoing mail, yes: large providers expect matching forward and reverse DNS and otherwise file the mail as spam or reject it.