421 4.4.5 Server busy
Meaning
The receiving mail server is too busy to accept your message at this time.
Common causes
- The recipient's server is handling a high volume of incoming connections.
- Your sending IP has hit a temporary rate limit set by the receiving server.
- The receiving server is undergoing maintenance.
How to fix
- Allow your mail transfer agent (MTA) to automatically retry the delivery.
- Ensure your sending volume is distributed evenly over time to avoid rate limiting.
- Monitor your sender reputation, as poor reputation can lead to stricter rate limits.
Provider notes
Gmail. Means 'Server busy, try again later.'
Example bounce
421 4.4.5 Server busy, try again later.
FAQ
- How long will the server be busy?
- Usually, the server will recover within a few minutes to a few hours. Your server will retry automatically.
- Should I slow down my sending rate?
- If you frequently encounter this error from a specific provider, reducing your connection and sending rate to that provider can help.