SMTP Error 550 – Authentication Required for Relay
If you receive the error message: "550 Authentication is required for relay" when trying to send email, it means your email client is not authenticating properly with the outgoing mail server.
Error Example
The message could not be sent because one of the recipients was rejected by the server.
Server Response: '550 Authentication is required for relay'
Server: 'mail.fast2host.com'
Protocol: SMTP
Port: 25
Secure(SSL): No
Error Number: 0x800CCC79
Cause
This error occurs when your email client attempts to send mail without authenticating against the SMTP server. Fast2Host requires authentication to prevent unauthorized use of our mail servers.
Solution – Enable SMTP Authentication
Follow these steps in Outlook to enable SMTP authentication:
- Open Outlook and go to File → Account Settings → Account Settings.
- Select your Fast2Host email account and click Change.
- Click More Settings.
- Go to the Outgoing Server tab.
- Tick the box: My outgoing server (SMTP) requires authentication.
- Select Use same settings as my incoming mail server.
- Click OK, then Next, and finally Finish.
Additional Notes
- Ensure you are using the correct SMTP server:
mail.fast2host.com. - Use your full email address and password for authentication.
- If port 25 is blocked by your ISP, use Fast2Host’s alternate SMTP ports (Windows hosting: 49, Linux hosting: 26).
- Enable SSL/TLS if your hosting plan supports secure connections.
✔ Tip: Always enable SMTP authentication in your email client.
This prevents relay errors and ensures your messages are delivered securely.