Temporary — will retry
SMTP 451 4.4.2
connection timed out
SMTP 451 4.4.2 means the connection dropped or timed out partway through the transaction. RFC 3463 defines X.4.2 as "bad connection", and Gmail returns it as "Timeout - closing connection." It is temporary, and your server will retry the message.
How it appears
451 4.4.2 Timeout - closing connection.
421 4.4.2 Connection dropped
What causes it
- A slow or unstable network between the two servers.
- Your server took too long to send a command or the message body.
- A firewall or proxy closed an idle connection.
How to fix it
In order — try the first one first.
- Let the retry happen. Most timeouts succeed on the next attempt.
- If one destination times out repeatedly, check network health and timeouts on your sending server.
- Look for firewalls or SMTP proxies that close long-running connections.
Common questions
- What does SMTP 451 4.4.2 mean?
- SMTP 451 4.4.2 means the connection dropped or timed out partway through the transaction. RFC 3463 defines X.4.2 as "bad connection", and Gmail returns it as "Timeout - closing connection." It is temporary, and your server will retry the message.
- Should I retry a 451 4.4.2 error?
- Yes, and your sending server does it automatically. 451 is a temporary deferral, and legitimate mail usually arrives on a retry within minutes or hours. Only investigate if it persists across many retries.
- How do I fix SMTP 451 4.4.2?
- Let the retry happen. Most timeouts succeed on the next attempt.
Other 451 replies
Last reviewed 2026-09-11