Log in to Plesk and choose "Mail" from the left side menu and click the email account name you want to change the settings for. In the next view select the "Spam Filter" tab.

With the check box "Switch on spam filtering for this email address" you can choose whether to use spam filtering or not.

In the next section you can select what to do with emails that are determined to be spam. The default is to mark the subject of the mail with a noticeable ***SPAM*** identifier. Another recommended option is to select "move spam to the Spam folder" which moves the detected spam directly to another directory without showing them in the Inbox. In that case you must remember to check the dedicated spam folder in case legitimate mail has been erroneously classified as spam.

Click "Show Advanced Settings" shows the options for spam filter sensitivity and both white and black lists.

By changing the sensitivity of the spam filter you can tune how easily a mail is classified as spam. The spam filter goes through the email source code and gives penalty points for features typically found in spam messages, and if at the end the score reaches the set threshold the mail is interpreted to be spam. Thus a lower limit means the mail is classified as spam more easily.

White and black lists are used for always allowing email from certain senders to come thought or for always blocking certain sender addresses.