Delayed delivery
Mail eventually arrives but takes much longer than normal due to greylisting, queues, or throttling.
Problem summary
Mail eventually arrives but takes much longer than normal due to greylisting, queues, or throttling.
Symptoms
- Messages arrive minutes to hours late
- Outbound queues grow during sends
- Recipients see delayed timestamps vs send time
Possible causes
- Provider throttling or greylisting of new/unknown senders
- Outbound concurrency or connection limits
- TLS or DNS lookup delays on the sending path
How to diagnose
- Compare queue age vs SMTP response codes (4xx vs 2xx)
- Segment delays by destination provider and sending IP
- Check whether delays correlate with volume spikes or new IPs
How to fix
- Reduce concurrency and smooth sending velocity
- Warm new IPs/domains before peak volume
- Raise provider limits only after reputation stabilizes
How to verify
- P95 delivery latency returns to baseline for major providers
- Queue depth stays flat during normal campaigns
Prevention
- Ramp volume intentionally
- Alert on rising average queue time
When to escalate
- Production mail is failing for a material share of recipients after remediation attempts