Emails not delivered
Messages never reach the recipient mailbox — rejected, discarded, or stuck without a successful delivery.
Problem summary
Messages never reach the recipient mailbox — rejected, discarded, or stuck without a successful delivery.
Symptoms
- Recipients report never receiving messages
- No inbox, promotions, or junk copy can be found
- Outbound logs show rejects, timeouts, or missing final DSN
Possible causes
- SMTP rejection or silent discard after acceptance
- Wrong MX or routing to an unintended system
- Blocklisting or severe reputation filtering
- Recipient address invalid or mailbox full treated as permanent failure
How to diagnose
- Confirm the exact recipient address and domain MX
- Inspect outbound SMTP response codes and DSN status
- Check Authentication-Results on any sample that did arrive elsewhere
- Test delivery to a known-good mailbox at the same provider
How to fix
- Fix MX, firewall, or TLS issues blocking the SMTP session
- Remediate authentication and reputation before retrying volume
- Suppress invalid addresses and correct routing configuration
How to verify
- Successful SMTP completion with a retrievable message copy
- Stable delivery to multiple mailboxes at the failing provider
Prevention
- Monitor bounce classes and deferral rates continuously
- Validate DNS and auth before major sends
When to escalate
- Production mail is failing for a material share of recipients after remediation attempts