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5.4.4 bounce: unable to route

5.4.4Unable to routePermanent: retrying will not help

Example
dsn=5.4.4, status=bounced (Host or domain name not found. Name service error for name=example.com type=A: Host not found)

What it means

X.4.4 is "Unable to route" in RFC 3463. The sending system got a definite answer from DNS (or from its routing tables) and that answer leaves no host to deliver to. Typical cases: the domain does not exist, its MX record names a host with no A or AAAA record, or there is no MX and no address record either. The example is the Postfix log text when the address lookup comes back as nonexistent.

Your own server or your relay produces this code. Microsoft 365 has its own DNS routing codes (5.4.1, 5.4.310, and 5.4.316) with separate pages. Compare 4.4.3, where the lookup failed and will be retried, and 5.1.2, which some servers use for the same "domain not valid" outcome.

Because a definite DNS answer is unlikely to change soon, 5.4.4 is the permanent form you normally get. A 4.4.4 shows up when the sender treats the missing route as temporary.

Likely causes

  • The MX record points to a hostname that has no A or AAAA record (for example after the mail host was renamed).
  • The domain has no MX and no address record at all, or has expired.
  • The MX record contains an IP address instead of a hostname, which is invalid and fails on many servers.
  • An internal relay or Exchange connector has no route configured for the destination domain.

How to fix it

  1. Look up the recipient domain’s MX records, then resolve each MX hostname to an IP address. A name that does not resolve is the usual culprit.
  2. If you own the domain, add the missing address record for the MX host, or point the MX at a hostname that exists, for example `example.com. IN MX 10 mail.example.com.` plus `mail.example.com. IN A 198.51.100.25`.
  3. Never put an IP address directly in the MX target; use a hostname with an A or AAAA record.
  4. If the failing route is inside your organisation, check send connectors, transport maps, or smart host settings for the domain.

Checks that apply

  • MX Lookup Shows the MX hostnames and whether they resolve, the most common gap behind a 5.4.4.
  • A Record Resolves each MX hostname to confirm it has an IPv4 address.
  • Whois Lookup Reveals an expired domain, which removes every route at once.
  • Domain Health Grades the whole domain, catching broken DNS records that leave mail with no route.
See the example diagnosed

The classification runs without network lookups and we never store your paste.

FAQ

Will my server retry a 5.4.4?
No. It is a permanent result, so the message is returned. Resend once the domain’s DNS is fixed.
Can a CNAME as the MX target cause this?
It can. RFC 2181 forbids MX targets that are CNAMEs, and some servers fail to route them.

All 191 bounce codes