SMTP 421 is a temporary failure often linked to throttling or server busy conditions. Learn common causes and remediation steps.
SMTP 421 is a temporary error code that usually means the receiving server is unavailable or is deferring mail due to load, rate limits, or policy controls.
The fix is usually operational: slow down sending, retry with backoff, and verify your sending reputation. If it persists, check whether your IP or domain is being rate-limited or blocked.