SMTP 4.4.1: connection timed out or refused

RFC 3463connectionaction: retry

4.4.1 means your server could not complete a connection to the receiving host. It is a transport problem rather than a policy one. Retry, but investigate if it is concentrated on one route or one destination.

What it looks like in the log

421 4.4.1 Connection timed out

451 4.4.1 Connection refused

Why it happens

• The receiving host is down or overloaded.
• A firewall between you and them is dropping the connection, sometimes silently.
• Your IP is being tarpitted, which is a reputation signal wearing a network costume.
• MX resolves to a host with no working A record.

What to do

• Retry on normal backoff. Most occurrences are genuinely transient.
• If it is one destination only, test from a different egress IP. If that works, it is reputation, not networking.
• If it is every destination, the problem is on your side: routing, firewall or DNS.

Related

tls-handshake

Classified as connection, action retry. Paste a response into the bounce classifier to check one against the same ruleset.

Written and maintained by Rastu Singh, an infrastructure engineer working on high-volume email in Tallinn.