5.3.2 bounce: system not accepting network messages
5.3.2System not accepting network messagesPermanent: retrying will not help
421 4.3.2 Service shutting down
What it means
X.3.2 is "System not accepting network messages" in RFC 3463. The host is up and answering, but it refuses all incoming mail for now. The usual reason is that the mail service is shutting down, starting up, or in maintenance mode. It often comes with basic code 421, which closes the connection.
MTAs commonly send 421 4.3.2 while they are stopping, and some appliances use it during updates or when refusing connections under heavy load. Note that this is not about you specifically: every sender gets the same answer. That separates it from 4.7.0 or 5.7.1 rejections, which target a sender, and from 4.4.1, where nothing answers at all.
The permanent 5.3.2 is uncommon. It points at a host that is configured never to accept mail from the network, such as a submission-only or send-only server that ended up in an MX record.
Likely causes
- The receiving MTA is restarting or shutting down for maintenance.
- The server is overloaded and refusing new connections until load drops.
- A send-only host was listed as an MX for the domain by mistake and rejects all inbound mail.
- A firewall or appliance put the mail service into a maintenance or bypass state.
How to fix it
- For 4.3.2, do nothing: your server retries automatically and delivery normally resumes once the service is back.
- For a persistent 5.3.2, look up the recipient domain’s MX records and check whether one of them points at a host that should not receive mail.
- As the domain owner, remove send-only hosts from the MX list and keep only servers configured to accept inbound mail.
- If you run the receiving server, check the MTA’s load limits and refusal thresholds and confirm the service is fully started.
Checks that apply
- MX Lookup Lists every MX host, so you can spot one that should not be receiving mail.
- SMTP Diagnostics Connects to the MX and shows whether it greets with 220 or refuses with a 421 shutdown message.
- TCP Port Test Checks port 25 on each MX host, telling a refusing service apart from a host that is not listening.
FAQ
- How long will my server retry after 4.3.2?
- Until its queue lifetime runs out, typically around five days. Maintenance usually ends long before that.
- Am I blocked if I see 4.3.2?
- No. It applies to every sender while the host is not accepting mail; it is not a block on you.