SMTP code 4.7.28
What the knowledge base's own tables say about this code, and which articles say it — a description of the reply, never a placement prediction.
- Cause
- Throttling or rate limiting
- Signature
- Deferrals rise with volume and concentrate at one provider: 421, "try again later", 4.7.28
- Test that tells causes apart
- Do deferrals grow with your sending rate? Is it one provider?
- Response
- Reduce concurrency and rate to that destination, and back off. See MTA delivery tuning and per-provider baselines