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~/errors / SMTP / Mail

SMTP: 450 4.1.8 Sender address rejected: Domain not found

The receiving server looked up the domain in the sender address and found neither an A nor an MX record, so it refuses the mail with reject_unknown_sender_domain. Typical sources are servers that send as root@hostname.localdomain.

daniele@mikweb: ~/errors — less smtp-450-4-1-8-sender-address-rejected-domain-not-found.md

What the message looks like

postfix/smtpd[3105]: NOQUEUE: reject: RCPT from unknown[203.0.113.30]: 450 4.1.8 <root@server1.localdomain>: Sender address rejected: Domain not found; from=<root@server1.localdomain> to=<admin@example.com> proto=ESMTP

Common causes

  • The sender address uses a made-up domain such as .local or .localdomain, often from cron or monitoring mail.
  • The sender domain really has no A or MX record, because of a typo or a domain not yet set up.
  • The DNS of the receiving server is broken or slow, so a good domain looks missing. The 4xx code means a temporary refusal.
  • A new or moved domain whose DNS has not propagated or was deleted by mistake.

Check

  1. Does the sender domain have an MX record?
    dig +short MX example.com
  2. Does it have an A record?
    dig +short A example.com
  3. What is the DNS status for the domain?
    dig example.com MX | grep -E 'status:'
  4. Which sender checks are active?
    postconf -n | grep -E 'smtpd_sender_restrictions|unknown_address_reject_code|myorigin'

Fix

  • If the domain is fake, change the sender. For your own server, set a real domain: myorigin = example.com in main.cf, or rewrite the sender with sender_canonical_maps.
  • For apps and cron, set the From address in their own settings to a real address in a domain that has DNS records.
  • Fix missing DNS: publish an A or MX record for the sender domain, and check the zone for typos.
  • If your own resolver is the problem, test it with dig example.com MX @1.1.1.1 and repair /etc/resolv.conf or the local resolver.
  • Only as a last resort, remove reject_unknown_sender_domain from smtpd_sender_restrictions. That check blocks a lot of junk.

FAQ

Why is it a 450 and not a 550?

Postfix uses unknown_address_reject_code, which defaults to 450 (try again). Senders retry, so a short DNS problem does not lose mail. Set it to 550 only if you want a hard refusal.

Will the mail still arrive?

A sending server keeps retrying for days. If the sender domain is fake, it will never succeed, so fix the address at the source.

daniele@mikweb: ~/blog — ls

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daniele@mikweb: ~/errors — ls

More errors

mail for example.com loops back to myselfPostfix: mail for example.com loops back to myself550 5.4.1 Recipient address rejected: Access deniedSMTP: 550 5.4.1 Recipient address rejected: Access denied550 5.7.1SMTP 550 5.7.1: relay access denied and policy rejections550 5.7.26SMTP 550 5.7.26 and 5.7.23: unauthenticated email554 5.7.1 Relay access deniedSMTP: 554 5.7.1 Relay access deniednot accepted due to domain's DMARC policy550 5.7.1 Unauthenticated email is not accepted due to domain's DMARC policy

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