Enabling return email DNS checking
When you enable return email DNS checking, your FortiGate unit will take the domain in the reply-to email address and send it to the configured DNS. If the domain does not exist, your FortiGate unit will treat the message as spam.
To enable return email DNS check
1. Go to Security Profiles > Email Filter > Profile.
2. The default email filter profile is presented. To edit another profile, select it from the drop down in the Edit Email Filter Profile title bar.
3. Select Enable Spam Detection and Filtering and select Apply
4. Under the heading Local Spam Filtering, select Return E-mail DNS Check.
5. Select Apply.
Select the edited email filter profile in a security policy, and the traffic controlled by the security policy will be scanned according to the settings you configured. You may select the email filter profile in more than one security policy if required.