The mail server is temporarily unavailable. This is a temporary error - try again later.
SMTP error 421 indicates that the receiving mail server is temporarily unable to process your request. Unlike 5xx errors, this is a temporary condition that should resolve itself.
The receiving server has too many connections and cannot accept more.
You're sending too many emails too quickly to this server.
The server intentionally delays first-time senders to filter spam.
The mail server is undergoing scheduled or emergency maintenance.
Most mail servers will automatically retry delivery. Wait 15-30 minutes before manually retrying.
If sending bulk emails, slow down your sending rate. Implement delays between messages.
Check if the recipient's email provider has reported any outages or maintenance windows.
Ensure your mail server has exponential backoff retry logic for temporary failures.
421 4.7.0 [x.x.x.x] Our system has detected an unusual rate of mail
421 Service not available, closing transmission channel
421 4.7.0 Try again later, closing connection