What This Error Means
The receiving server is temporarily refusing connections because you've exceeded their rate limits. This is a temporary error and you should retry after a delay.
Common Causes
- 1Sending too many emails too quickly
- 2Multiple connections from the same IP
- 3Shared IP with other high-volume senders
- 4Aggressive retry behavior
How to Fix It
- 1Implement exponential backoff for retries
- 2Reduce sending rate and spread emails over time
- 3Use multiple IPs for high-volume sending
- 4Check if your ESP provides dedicated IPs
Quick Reference
- Error Code
- 421 4.7.0
- Category
- Temporary Failure
- Severity
- Medium
- Should Retry?
- Yes, with backoff
View All SMTP Errors