• The server you are connected to is using a certificate that cannot be verified.
• The target principal name is incorrect.

Solution:

• 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.