SMTP codes
SMTP 421 Service not available
The server is closing the connection: overloaded, shutting down, or limiting you (too many connections or messages). Temporary.
- Class
- 4xx · Temporary failure
- Retried?
- Yes, the sending server retries
- Defined in
- RFC 5321
What to do
- Let your server retry; it will do so automatically.
- If it repeats for hours, lower the sending rate and the number of parallel connections to that domain.
- Check your IP on blacklists: some servers answer 421 to listed senders.
How it appears in a bounce
421 Service not available
Paste the full bounce into the decoder — it finds every code and recipient and says what to check.