421: service not available

What it means

A 421 says the server cannot carry on with the session. RFC 5321 allows it in reply to any command when the service has to shut down, and servers also send it when they are overloaded or want a client to slow down. Because it is a 4xx reply, nothing is lost: the sending server keeps the message in its queue and retries. Large mailbox providers send 421 with enhanced codes and text of their own to rate-limit senders, so the words after 421 tell you whether this is maintenance or a reaction to how you send. If every retry meets a 421 until the retry period runs out, the message is finally returned as undeliverable.

Common causes

• The receiving server is restarting, shutting down or in maintenance.
• The server is overloaded or has reached its limit of connections.
• A provider is rate-limiting your IP address or domain because of volume or reputation.

How to fix it

1. 01Let the sending server retry; a single 421 is part of normal mail traffic.
2. 02Read the enhanced code and the text after 421, which often say whether it is a rate limit.
3. 03If it keeps happening with one provider, send to it more slowly and review your sending reputation.
4. 04If the text mentions reputation or a block, check your IP and domain on blocklists.

Sources

What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.

• RFC 5321, section 4.2.3: Reply Codes in Numeric Orderrfc-editor.org
• RFC 5321, section 3.8: Terminating Sessions and Connectionsrfc-editor.org

Further reading

• SMTP error codes explained
• Gmail 421 4.7.0 decoded