Log in to the Plesk control Panel using the link https://yourdomain.com:8443.

Click on the 4PSA Spam Guardian  under "Main Menu".

Click on the icon to change spam security settings for the domain.

 You can change the spam engine settings and click "Update".

To whitelist a domain you can enter the email id of that domain and click on "Add".

Similary to blacklist  a domain you can  give*@domainname.com or give the specific email id of that domain and click on   "Add " to update.

Logout after you finish.