What 421 4.4.2 means
4.4.2 is RFC 3463's 'Bad connection': the connection was established, but the message transaction couldn't complete because of a timeout or poor connection quality. RFC 3463 says it's only useful as a transient error, so the message should be retried.
Gmail documents its version as 451 4.4.2 'Timeout - closing connection'. Microsoft Exchange often shows 421 4.4.2 when a socket drops between servers. Either way, nothing was rejected; the transfer just didn't finish.
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 4.4.2 Timeout - closing connection.Why cold emailers see 421 4.4.2
- A slow or overloaded sending server that took too long between SMTP commands, so the receiver timed out.
- Network trouble between the two servers: packet loss, a firewall or proxy cutting long connections, or a TLS handshake that stalled.
- Very large messages (big attachments or embedded images) on a slow link.
- Sending too many messages over one connection, or keeping idle connections open too long.
How to fix 421 4.4.2
- Let your server retry. A one-off 4.4.2 needs no action.
- If it repeats for one destination, check the sending host's network path and any firewall or antivirus that inspects port 587/465/25 traffic.
- Keep cold emails small: plain text or light HTML, no attachments, no large images.
- If you run your own MTA, check its SMTP timeouts and CPU/memory load at the times the errors appear.
Will it retry on its own?
Retry. Don't suppress anyone over a 4.4.2. If it never succeeds after the full retry window, the message expires (typically reported as 4.4.7 'Delivery time expired') and you should investigate your own server's connectivity.
Reading the code
The basic code 421 means "Service not available, closing connection (temporary)" under RFC 5321. In 4.4.2, the 4 is the class (persistent transient failure) and the 4 is the subject (network and routing). The detail number 2 is registered in RFC 3463 / the IANA registry as "Bad connection".
- 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 4.4.2
- Bad connection
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.