An SMTP 4xx code is a temporary failure: the receiving server is saying « not now, try again later ». The address is usually valid, but something on the destination side is preventing delivery right now — full mailbox, busy server, brief greylisting or a soft rate limit. Mailpro retries 4xx automatically before marking them.
Soft bounce in one line 4xx = soft bounce. Don’t purge the address — let the system retry. Only worry if the same 4xx repeats over several days.
The most common 4xx errors
| Code | Typical message | Likely cause |
|---|---|---|
| 421 | Service not available, try later | Server busy or maintenance. |
| 421 4.7.0 | Too many connections | Rate limit hit by the receiver. |
| 450 | Mailbox temporarily unavailable | User exists, server temporarily refusing. |
| 451 | Local error in processing | Internal issue on the destination server. |
| 451 4.3.0 | Greylisting | Greylisting in place; the retry will succeed. |
| 452 | Insufficient system storage | Mailbox or server short on space. |
What you should do
For a one-off 4xx, do nothing — Mailpro retries the message at increasing intervals until it goes through or gets escalated to a hard bounce. If the same address keeps failing for several days, it usually means a full mailbox or an abandoned account, and removing it is the right move.
Don’t mass-resend manually Sending the same campaign twice on a 4xx burst can trigger a 5xx and damage your reputation. Trust the automatic retry.
Understand bounces and authentication
Read the complete bounces guide, see how soft bounces work and the most common SMTP errors.