Reference
SMTP error lookup
The first digit is what matters. 4xx is temporary — the sender will retry, often for days. 5xx is permanent — the message bounces.
Paste a code or part of a message to find it.
Nothing matches that. The code may be specific to the receiving server — the text of its reply is usually the better clue.
Codes Corsair returns#
| Code | Meaning | What to do |
|---|---|---|
| 421 | Too many errors, or too many failed logins from this address | The IP is temporarily banned. Wait an hour. |
| 450 | Mailbox temporarily unavailable | Retry. |
| 451 | Daily sending limit reached | The account is over its outbound allowance for the day. |
| 452 | Storage quota exceeded, or recipient's daily inbound limit reached | Free space or upgrade; the sender will retry. |
| 501 | Malformed address or syntax | The sending client has a bug. |
| 503 | Commands out of order | Usually a client that pipelined without checking for support. |
| 530 | Authentication required | Submission needs AUTH. Check the client is using 587 or 465, not 25. |
| 535 | Credentials invalid | Wrong mailbox password. Note this is not the panel password. |
| 538 | Encryption required | The client tried to authenticate before STARTTLS. |
| 550 | No such user, or relay denied | The address does not exist here, or the domain is not hosted here. |
| 552 | Message too large | Over MAX_MESSAGE_BYTES. |
| 554 | No valid recipients | Every recipient was rejected. |
Codes you will see from others#
| Code | Usually means |
|---|---|
| 421 4.7.0 | Rate-limited by the receiver. Slow down; this is not an error in your configuration. |
| 450 4.2.0 | Greylisting. The first attempt is always refused; the retry is accepted. Nothing to fix. |
| 550 5.7.1 | Blocked on policy — reputation, SPF, or DMARC. The text usually names which. |
| 550 5.7.26 | Unauthenticated: no aligned SPF or DKIM. Your DNS is wrong or incomplete. |
| 554 5.7.1 | Listed on a blocklist. The text usually names it. |
Reading a bounce#
A bounce carries three parts: a human-readable explanation, a machine-readable message/delivery-status report, and the original headers. The Diagnostic-Code line in the middle part is the remote server's verbatim reply — that is the one to act on, not the summary at the top.