Delayed delivery
Mail eventually arrives but takes much longer than normal due to greylisting, queues, or throttling.
- Reading time:
- 5 min
- Difficulty:
- Intermediate
- Estimated duration:
- 5 min
- Prerequisites:
- None
- Last updated:
- 2026-07-15
- Author:
- empire
Problem summary
Mail eventually arrives but takes much longer than normal due to greylisting, queues, or throttling.
Severity: Medium
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
Related Learn Articles
Related Problems
- Deferred messagesReceivers return temporary SMTP failures (4xx), forcing retries instead of immediate acceptance.
- Emails not deliveredMessages never reach the recipient mailbox — rejected, discarded, or stuck without a successful delivery.
- Hard bouncePermanent delivery failure (typically 5xx) because the address or domain cannot accept mail.
- Soft bounceTemporary delivery failure that may succeed later — full mailboxes, greylisting, or transient outages.