What does 421 mean?

SMTP 421 indicates temporary service unavailability:

What it means:

The receiving server cannot process your request right now. The connection is being closed. You should try again later.

Common causes:

Rate limiting: You are sending too fast. Server overload: Too many connections. Scheduled maintenance: Server temporarily down. Reputation-based throttling: Sender being slowed down.

Typical message examples:

"421 Try again later" "421 Too many connections" "421 Service temporarily unavailable"

Response:

Queue for retry with appropriate backoff. Do not suppress the address. Monitor for patterns suggesting deeper issues.

421 is "come back later." The server is temporarily unable to serve you.