Hard bounces (5xx) are permanent failures. Remove immediately after one occurrence.
| Code | Meaning |
|---|---|
| 550 5.1.1 | Mailbox does not exist |
| 550 5.1.2 | Domain doesn't exist |
| 553 | Mailbox name not allowed |
| 554 | Transaction failed |
Soft bounces (4xx) are temporary. MTAs auto-retry with exponential backoff.
| Code | Meaning |
|---|---|
| 421 | Try again later (rate limiting) |
| 450 | Mailbox temporarily unavailable |
| 451 | Greylisting / local error |
| 452 | Insufficient storage |
🎯 Target: Keep bounce rates below 2% across all sends.
Feedback Loops (FBLs): Let you know when recipients click "Report Spam."
- Microsoft JMRP — Register via SNDS portal
- Yahoo CFL — Domain-based, requires DKIM
- Gmail — No traditional FBL; use Postmaster Tools instead
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