SMTP errors fall into two families: 4xx codes are temporary (retry), 5xx codes are permanent (don’t retry until you fix the cause). Reading the three-digit code plus the text that follows tells you exactly what happened — mailbox issue, authentication problem, content rejection or rate limit.
How to read a code First digit: 4 = soft, 5 = hard. Second digit: 1 = syntax, 2 = response, 3 = mail system, 4 = network, 5 = mail itself, 7 = security/policy.
The most common SMTP errors
| Code | Meaning | First action |
|---|---|---|
| 421 | Service not available, try later | Wait, then resend. |
| 450 | Mailbox temporarily unavailable | Retry later. |
| 451 | Local error, processing failed | Retry later. |
| 452 | Insufficient system storage | Retry later. |
| 550 | Mailbox unavailable / user unknown | Remove the address. |
| 551 | User not local; please try forwarding | Remove or use the suggested address. |
| 552 | Storage exceeded / message size | Lighten the message. |
| 553 | Mailbox name not allowed | Check the From and recipient syntax. |
| 554 | Transaction failed / blocked | Check authentication, reputation and content. |
What to check when you see a 5xx
Verify your authentication (SPF, DKIM, DMARC), the IP reputation, the content of the message and the validity of the recipient address. Most 5xx errors at scale come from one of those four causes.
Don’t mass-retry 5xx 5xx codes mean the server intentionally refused. Repeating the send only damages your sender reputation.
Dive deeper into SMTP errors
Read the common SMTP errors guide, see the focus on SMTP 554, and learn how Mailpro manages bounces — hard and soft.