What it is
Bounces and delivery-status notifications describe failed or delayed recipients with SMTP replies and structured enhanced status codes.
Why it matters
Permanent failures need suppression; temporary failures need bounded retry. Treating both alike either wastes capacity or drops recoverable mail.
How to implement
Parse machine-readable status data, preserve the original recipient correlation, distinguish 4.x.x from 5.x.x outcomes, and apply policy by failure class rather than free-text wording.
Common mistakes
- Matching only English diagnostic phrases.
- Suppressing an address after one ambiguous timeout.
- Sending bounces to an unverified visible From address.
Verification
Replay fixtures for user unknown, mailbox full, policy block, DNS failure, timeout, and delayed delivery. Each must follow the intended retry or suppression path.