Temporary (4xx) — MTA will retry:
| Code | Meaning |
|---|---|
| 421 | Service unavailable / rate limiting |
| 450 | Mailbox temporarily unavailable |
| 451 | Local error / greylisting |
| 452 | Insufficient storage |
Permanent (5xx) — Delivery failed:
| Code | Meaning |
|---|---|
| 550 5.1.1 | Mailbox does not exist |
| 550 5.7.1 | Policy rejection (spam/DMARC) |
| 550 5.7.26 | DMARC policy failure (Gmail) |
| 552 | Message too large |
Gmail-specific:
- 421 4.7.26 — SPF/DKIM failure (temporary)
- 421 4.7.28 — Rate limiting ("unusual rate")
- 550 5.7.26 — DMARC fail (permanent rejection since 2024)
Microsoft-specific:
- 550 5.7.515 — Non-compliant bulk sender (since May 2025)
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