Diagnosing email transmission failures requires inspecting raw SMTP enhanced status codes (RFC 3463). Distinguishing temporary connection deferrals from fatal mailbox errors prevents unnecessary IP blacklisting.
1. Essential SMTP Response Code Reference
| SMTP Code | Classification | Meaning | MTA Action |
|---|---|---|---|
550 5.1.1 | Permanent Failure (Hard) | User unknown / mailbox does not exist | Immediately suppress address from future sends |
451 4.7.1 | Temporary Failure (Soft) | Greylisting / Temporary anti-spam delay | Retry delivery after 15–30 minute interval |
421 4.7.0 | Temporary Failure (Soft) | Too many concurrent connections / Rate limited | Throttle concurrency down by 50% |
554 5.7.1 | Permanent Failure (Hard) | Relay access denied / IP blacklisted | Halt sending on IP; trigger blacklist audit |