If you don't see a link above that says My Services then you will need to login, then...
Click on My Services, then click the The square green icon to the right of your domain. Click the Logon to Plesk button.
Click the Mail tab.
Click on the email address for which you want to adjust the spam filter settings.
Click on Spam Filter
Click on Advanced Settings
Adjust the spam score as needed. A higher number will less likely to trap a real email message and will let more spam thru. A lower number will catch more spam but will be more likely to trap wanted email. The most commonly used number is 3. We don't recommend going any lower than 2. You do not need to use whole numbers, for example 3.2 is acceptable.