- The server you are connected to is using a certificate that cannot be verified.
- The target principal name is incorrect.
- Click on the View Certificate
- Check the Issued to value
- Now go to Microsoft Outlook and navigate to: File | Account Setting | Account Setting
- Select your e-mail account and click on the Change button.
- Change the values of the Incoming and Outgoing (SMTP) server address so it matches the name on the Certificate. mail.domain.com
- Click Next and Finish
The warning message should be gone.