Deferred messages
Receivers return temporary SMTP failures (4xx), forcing retries instead of immediate acceptance.
Problem summary
Receivers return temporary SMTP failures (4xx), forcing retries instead of immediate acceptance.
Symptoms
- High volume of 4xx SMTP responses
- Messages retry repeatedly before succeeding or bouncing
- Provider responses mention rate limits or try again later
Possible causes
- Per-IP or per-domain rate limits
- Poor or unknown sender reputation
- Greylisting policies for first contact
- Transient receiver-side capacity issues
How to diagnose
- Group deferral codes by destination and sending identity
- Confirm whether authentication failures accompany deferrals
- Compare deferral rate before/after volume or IP changes
How to fix
- Slow sending and respect provider rate guidance
- Improve reputation signals before increasing concurrency
- Fix auth/DNS issues that may be contributing to distrust
How to verify
- Deferral rate drops and first-attempt acceptance rises
- Retries complete without exhausting bounce policies
Prevention
- Cap burst volume on new IPs
- Monitor 4xx ratios per provider
When to escalate
- Production mail is failing for a material share of recipients after remediation attempts