What 451 4.4.0 means
4.4.0 is RFC 3463's 'Other or undefined network or routing status': something went wrong with networking or routing, without a more specific code. With a 451 basic code, it's a temporary failure and the message stays in the queue for another attempt.
The most common form is Microsoft Exchange's '451 4.4.0 DNS query failed', meaning the server couldn't resolve the recipient domain's MX records to find the next hop. It's also used for 'primary target IP address responded with' errors, where Exchange quotes the remote server's reply after the code; in that case the quoted reply is the real error.
What the bounce looks like
The wording providers document for this code. Your bounce may add an IP address, a session ID or a help link after it.
451 4.4.0 DNS query failedExchange queue error: the next-hop DNS lookup failed.
Why cold emailers see 451 4.4.0
- The recipient domain's DNS servers are down, slow, or returning malformed answers (some ignore AAAA queries).
- The sending server's own DNS resolver is broken or misconfigured.
- The recipient domain has no MX record or its MX points to a host that doesn't resolve.
- A remote server rejected the connection, and Exchange wrapped that reply in a 4.4.0 status.
How to fix 451 4.4.0
- Look up the recipient domain's MX records yourself (dig MX example.com or an online lookup). If there are none or they don't resolve, the domain can't receive mail.
- If it fails for every domain, fix the sending server's DNS resolver.
- If the error quotes a remote response ('responded with: 421 ...' or '550 ...'), look up that inner code instead.
- For cold email, verify lead domains before sending. A domain with no working MX is a guaranteed bounce.
Will it retry on its own?
Retry for transient DNS trouble. If the recipient domain has no MX record at all, treat the address as invalid and suppress it; it will never accept mail.
Reading the code
The basic code 451 means "Local error in processing, action aborted (temporary)" under RFC 5321. In 4.4.0, the 4 is the class (persistent transient failure) and the 4 is the subject (network and routing). The detail number 0 is registered in RFC 3463 / the IANA registry as "Other or undefined network or routing status".
- Class 4
- Persistent transient failure
- A temporary condition delayed delivery. The message itself is valid, and sending again later may succeed.
- Subject 4
- Network and routing
- A problem with networking, DNS or routing between servers.
- Detail 4.4.0
- Other or undefined network or routing status
Check the rest of your setup
Paste the headers of a message that did get through into the email header analyzer to confirm SPF, DKIM and DMARC pass, and check your domain with the SPF, DKIM & DMARC checker. The deliverability checklist covers the rest of a healthy sending setup.
Got a different bounce? Paste it into the SMTP error code lookup to decode it.