A 550 relay rejection means a mail server is refusing to pass the message on to the requested destination. Check which server issued the response before changing the email application’s settings or the receiving domain’s routing.
What is the 550 Relay Not Permitted error?
You may see 550 Relay not permitted or 550 5.7.1 Unable to relay. A mail client can encounter this during sending, while a bounce can report a rejection from a different server later in delivery.
What causes this error?
Common causes include missing SMTP authentication, incorrect credentials, using the wrong outgoing server, or a receiving server that does not recognise the recipient domain as one it should handle. A mail-routing or forwarding problem can therefore need a different fix from a client login problem.
Fixes for the 550 Relay Not Permitted error
- Identify the rejecting server. Read the complete error or bounce, noting the server hostname, time and recipient. Keep credentials out of any shared diagnostic information.
- Check the outgoing settings. Use the SMTP hostname supplied by your mail provider, enable SMTP authentication, and use the complete mailbox username and correct password.
- Check encryption and ports. Use the provider’s supported submission settings, commonly port 587 with STARTTLS or 465 with implicit TLS. These modes are different; match the application’s settings to the chosen port.
- Check the sender address. Confirm that the authenticated account is permitted to send using the selected From address.
- Check incoming routing if the rejection comes from the recipient server. The administrator should verify the domain’s MX records, mailbox existence and local or remote mail-routing configuration. The receiving MX server can differ from the outgoing submission server.
- Collect evidence if the problem remains. Give the administrator the rejection text and message time so they can inspect the relevant mail log.
Do not turn the server into an open relay or trust arbitrary IP addresses to work around this error. SPF, DKIM and reputation issues need their own diagnostic evidence; adding an invented SPF include is not a relay-authentication fix.
Further investigation
After correcting the identified cause, send a new test message and check delivery. If only a forwarded message fails, investigate that forwarding route and the downstream provider’s requirements.