Emails not delivered

Messages never reach the recipient mailbox — rejected, discarded, or stuck without a successful delivery.

Severity: High

• Recipients report never receiving messages
• No inbox, promotions, or junk copy can be found
• Outbound logs show rejects, timeouts, or missing final DSN

• 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

• 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

• Fix MX, firewall, or TLS issues blocking the SMTP session
• Remediate authentication and reputation before retrying volume
• Suppress invalid addresses and correct routing configuration

• Successful SMTP completion with a retrievable message copy
• Stable delivery to multiple mailboxes at the failing provider

• Monitor bounce classes and deferral rates continuously
• Validate DNS and auth before major sends

• Production mail is failing for a material share of recipients after remediation attempts