4.4.0 bounce: other network or routing status
4.4.0Other or undefined network or routing statusTemporary: the sender retries
451 4.4.0 Primary target IP address responded with: "421 4.4.2 Connection dropped." Attempted failover to alternate host, but that did not succeed.
What it means
X.4.0 is "Other or undefined network or routing status" in RFC 3463. Subject 4 covers the path between mail servers: DNS routing, TCP connections, and the network itself. Detail 0 means the server saw a network-level failure but did not say which one, so it is broader than 4.4.1 (no answer) or 4.4.2 (connection dropped).
Microsoft Exchange and Exchange Online use 4.4.0 a lot, often wrapping the real error from the next hop and noting that failover to another host also failed, which is the shape of the example. Other MTAs use it for DNS or connection errors they cannot map. It is usually generated by the sending side, in a delay notice or in the final DSN, not by the recipient’s server.
The 4 class is normal here because network trouble is expected to clear; your server keeps retrying. A 5.4.0 means the sender gave up or found a condition it considers permanent, such as a DNS answer that will not change.
Likely causes
- The recipient MX dropped or refused the connection and no backup MX accepted it either.
- A DNS lookup for the recipient domain returned an error the sender could not classify.
- A smart host or connector in your own organisation points at a host that is down.
- TLS or firewall issues between the two servers broke the session in a way the sender mapped to a generic network error.
How to fix it
- Read the full text after 4.4.0; Exchange usually quotes the response from the remote host, which is the real error.
- Check the recipient domain’s MX records and test port 25 on each host to see whether any accepts connections.
- If you relay through a smart host or an Exchange connector, confirm its target host and port are correct and reachable.
- Let the queue retry if the remote side is simply having an outage, and tell the recipient through another channel if it lasts more than a few hours.
Checks that apply
- MX Lookup Lists the recipient’s mail hosts in priority order, the hosts your server keeps failing to reach.
- TCP Port Test Tests port 25 on each MX host, showing which one refuses or drops connections.
- SMTP Diagnostics Runs a full SMTP greeting and STARTTLS check to catch failures after the TCP connection succeeds.
FAQ
- Has my message been lost?
- No. A 4.4.0 notice means it is still in your server’s queue and will be retried.
- When does 4.4.0 turn into a failure?
- When the queue lifetime expires, often after two to five days, you get a final bounce, typically with 4.4.7 or 5.4.0.