Data lookups

Email error codes

What SMTP 421, 550 and 554 are telling you, and what to do about it.

All lookups

28 entries.

CodeMeaningWhat to do
211 System status message. Informational only.
214 Help message. Informational only.
220 Server ready. Normal greeting when a connection opens.
221 Closing connection. Normal end of session.
235 Authentication successful. Your credentials were accepted.
250 Requested action completed. The normal success reply — the message was accepted.
251 User not local; will forward. The message is being relayed onward.
354 Start mail input. The server is ready for the message body.
421 Service not available, closing channel. Temporary. The server is overloaded or throttling you — retry later, and slow your sending rate.
450 Mailbox unavailable (busy or temporarily blocked). Temporary. Often greylisting; it usually succeeds on retry.
451 Local error in processing. Temporary server-side fault. Retry; if it persists, check the recipient server status.
452 Insufficient system storage. Temporary. The mailbox or server is full.
454 Temporary authentication failure. Often a TLS or OAuth token problem — re-authenticate.
500 Syntax error, command unrecognised. The client sent something invalid. Check your mail software.
501 Syntax error in parameters or arguments. Usually a malformed email address.
502 Command not implemented. The server does not support that command.
503 Bad sequence of commands. Commands sent in the wrong order — often authenticating too late.
504 Command parameter not implemented. Frequently an unsupported authentication mechanism.
521 Server does not accept mail. This host is not a mail server.
530 Authentication required. You must log in before sending. Check SMTP credentials.
535 Authentication credentials invalid. Wrong username or password, or an app password is needed.
541 Message rejected by the recipient's filters. Content looked like spam. Review wording, links and sending reputation.
550 Mailbox unavailable / relay denied. Permanent. The address does not exist, or you are not allowed to relay. The most common bounce.
551 User not local. Permanent. The server will not forward for that address.
552 Storage allocation exceeded. The recipient mailbox is full, or the message is too large.
553 Mailbox name not allowed. The address is malformed or rejected by policy.
554 Transaction failed. Permanent, catch-all rejection — commonly spam blocking or a blocklisted IP. Check your sending IP reputation and SPF/DKIM/DMARC.
556 Domain does not accept mail. The recipient domain has a null MX record.
A 4xx code is temporary — the sending server will keep retrying, usually for a few days. A 5xx is permanent and produces a bounce.