SMTP errors: fix 535 authentication and TLS failures
Diagnose SMTP 535, 530, 421, 451, 452 and 550 responses, connection timeouts and TLS configuration errors.
What SMTP responses mean
Your mail library may show only a code and the final server message. Codes beginning with 4 are temporary and may be retried; codes beginning with 5 require a configuration or message change.
535Authentication failed+
Meaning: The username/password is wrong or the credential was revoked.
What to do: Generate a new credential for the correct inbox and update both values.
530Authentication is required+
Meaning: The connection tried to send before authenticating, or AUTH was attempted without TLS.
What to do: Enable SMTP authentication and STARTTLS, or use implicit TLS on port 465.
421Too many SMTP connections from this address+
Meaning: Your source IP opened too many connections in a short period.
What to do: Reuse connections, reduce parallel workers and retry with backoff.
451SMTP message rate exceeded+
Meaning: Messages are arriving faster than the temporary SMTP rate allows.
What to do: Slow the sender and retry later with exponential backoff.
452SMTPBox monthly sandbox allowance reached+
Meaning: The Free workspace has used its monthly 2,500-message allowance.
What to do: Wait for the next monthly period or ask the owner to upgrade.
550Too many recipients+
Meaning: One message contains more recipients than the sandbox accepts.
What to do: Reduce the To, Cc and Bcc recipient count and resend.
550Unknown SMTPBox receiving address+
Meaning: A generated inbound address is missing, disabled or incorrect.
What to do: Copy the receiving domain again from the dashboard.
451Message could not be safely persisted+
Meaning: SMTPBox could not safely store the message at that moment.
What to do: Retry later. If it continues, keep the timestamp and contact support.
Connection refused usually means the host or port is wrong. A timeout often points to a firewall blocking outbound SMTP. Certificate or handshake failures usually mean the TLS mode does not match the port.
Updated