Web Technician OnlineWhat does this mean?
421 indicates that the service is unavailable and the connection is being closed. The complete provider response may explain maintenance, resource pressure or a service policy. A temporary code does not establish that your recipient address is wrong.
What should I check first?
Save the full reply and identify which server returned it. Check whether your sending client can connect at all and whether webmail has the same problem. If only one client fails, compare its configured hostname, port and supported security method with provider instructions.
How can I diagnose the cause?
Inspect the provider status page and queue logs at the failure time. Note whether every message is affected or only a burst of traffic. A provider may temporarily limit an account, IP or destination; the reply and logs are more useful than changing random SMTP ports.
How do I fix it safely?
Allow the supported retry process and address any named account or service limit. If settings changed recently, restore the documented SMTP configuration after saving a copy. Do not bypass required authentication, disable TLS or repeatedly reconnect at high speed, which can worsen throttling.
Verify the fix and know when to contact your provider
Check that queued mail later completed or failed before manually sending duplicates. Contact the provider when service availability remains uncertain or the reply names a restriction you cannot resolve. Provide timestamps, server hostname, message ID if assigned and full redacted response, with credentials removed.
Work through these checks in order
- Compare the configured SMTP hostname, port and security mode with the provider’s current instructions. If webmail succeeds, investigate the client path before changing domain records.
- Record which server closed the connection and check its service status or account limits. Avoid rapid reconnect loops that can create additional throttling.
- Confirm whether the message ever entered a queue. Give the provider the connection time and full redacted reply if no message ID was assigned, and verify final delivery before manual resending.
Which tool can help?
DNS tools show one resolver’s public answers. Record explainers do not authenticate a message, and calculators do not monitor a server. Use the evidence alongside your provider’s logs.