451 4.4.2: Connection timed out
451 4.4.2 means the connection to the receiving server dropped or timed out before the message was complete. It is temporary and usually a network problem, not a judgement about your mail.
- Bounce type
- Soft bounce (temporary)
- Usually caused on
- Either side, read the text
- Terlido report label
- Temporary failure
What the standard says
RFC 5321 defines 451 as "requested action aborted: local error in processing". RFC 3463 defines X.4.2 as "Bad connection": the connection was made but it dropped or timed out before the transaction finished. Subject 4 covers the network and routing, which is why the cause is rarely the content.
What the mailbox providers say
- Gmail (451 4.4.2): “Timeout - closing connection.” Source
Common causes
- A slow or overloaded sending server that took too long to send the data.
- A network problem between the two servers, for example packet loss.
- A very large message that needs more time than the connection allows.
- A receiving server that closes idle connections quickly and cuts a slow sender off.
- A sending server with a slow link, for example a small VPS under load, which cannot push data fast enough.
- A firewall or security gateway on the receiving side that cuts connections it considers slow.
How to fix it
- Retry later. Most cases go away on their own.
- Reduce the size of the message, because small messages finish before a timeout.
- If you run your own SMTP server, check its network path, its load and its DNS settings.
- Check that your server has working reverse DNS, since some receivers drop connections from hosts that look misconfigured.
- Send at a calmer pace if you run your own server, so that fewer connections compete for the same link.
- If the code appears for one receiver only, retry later and compare with others before you change settings.
- Avoid sending several large messages over the same connection, and let the sending system open a new connection when needed.
Is it a deliverability problem?
Timeouts are normally not reputation signals. A pattern that only affects one receiver is worth a look at your connection and server setup. If you send through Google or Microsoft mailboxes, the cause is more likely on the network side than in your own configuration, and a timeout mostly has nothing to do with your domain's standing. For your own server, keep an eye on repeated timeouts, because a receiver that drops your connections often enough may start to treat the server as unreliable.
How Terlido handles it
When a bounce message for this code arrives in a connected mailbox, Terlido reads the status code. An enhanced code that starts with 4 is a soft bounce. In the campaign Reports tab this one is grouped as "Temporary failure".
On a soft bounce Terlido logs the event on the lead's activity. The lead is not stopped, and the sequence continues at its next scheduled step.
Check the domain behind the bounce
The free domain checker runs live SPF, DKIM, DMARC, MX and blocklist lookups and explains each result in plain English.
Free tools that help
- Email size checkerMeasure raw HTML and text byte size.
- Reverse DNS checkerCheck PTR and forward matching for a public IP.
Related error codes
- 451 4.3.0: Temporary mail system problem
- 552 5.3.4: Message too big
- 550 5.7.25: Sending IP has no reverse DNS record