Simple Mail Transfer Protocol (SMTP) status codes provide granular feedback on MTA transaction state. Correctly distinguishing between transient 4xx deferrals and permanent 5xx rejections is vital for maintaining sender reputation.
1. Critical SMTP & Enhanced Status Codes
| Code | Enhanced Code | Classification | Recommended MTA Action |
|---|---|---|---|
| 421 | 4.7.0 | Temporary Rate Limit / Deferral | Backoff exponential retry; hold queue for 15–60 min |
| 450 | 4.2.2 | Mailbox Temporarily Full | Retry up to 72 hours; do not immediately suppress |
| 550 | 5.1.1 | Unknown User / Non-Existent Address | Immediate suppression; permanent Hard Bounce |
| 554 | 5.7.1 | Message Rejected by Content/Spam Filter | Review email content, SPF/DKIM headers, and domain reputation |
⚡ Suppression List Rule
Any recipient triggering an RFC 5.1.1 or 5.1.2 response must be automatically added to an immutable suppression list to prevent repeat transmission.