MailFixIt

Generic SMTP delivery response

Understand SMTP 4.4.2: Connection timed out

The servers connected, but the SMTP transaction could not finish because the connection timed out or became unreliable.

Problem description

A representative response is:

421 4.4.2 Connection timed out.
Is this temporary or permanent?

This is temporary. Preserve the queue and retry with backoff while checking the failed stage of the connection.

What this response means

Standard 4.4.2 Bad connection means the connection was established but the SMTP transaction did not finish. The log should show whether it stalled before the greeting, during STARTTLS, after a command, or while transferring message data.

What to do

  1. Keep the message queued and use normal retries with increasing delays.
  2. Inspect the SMTP transcript to find the last successful response and the exact timeout.
  3. Check packet loss, firewall session limits, MTU problems, TLS negotiation, proxy timeouts, and server load on the real path.
  4. Compare small and large test messages only if logs suggest the failure happens during data transfer.
  5. If one destination host repeatedly fails while others work, give its operator the timestamps, source IP, and connection evidence.

Do not increase every timeout blindly; that can hide a broken service and consume sending resources.

Who can fix it

The operator of the sender, intermediate gateway, receiver, or network segment where the session stalls must correct it.

Use the TLS or MX guide only when the transcript specifically points to that layer.

Frequently asked questions

What does SMTP 4.4.2: Connection timed out mean?

The SMTP connection started but did not complete the transaction because of a timeout, connection loss, or inadequate connection quality.

Can MailFixIt prove this entire bounce from public DNS?

No. The current checker can evaluate selected public DNS configuration, but it cannot recreate recipient state, credentials, reputation, message content, or a historical provider SMTP decision.

Related email diagnostics

SMTP 550 5.7.26: Multiple authentication checks failedUnderstand SMTP 5.7.26 and separate SPF, DKIM, DMARC, and alignment failures before changing email DNS.

Primary sources: IANA SMTP Enhanced Status Codes.