If you are getting a lot of spam in Plesk and looking to reduce it -
following these steps:
Login to Plesk > domain.com > Mail > Preferences > remove the 'Catch-all' email configured. Doing this reduces the spams by 50%.
Login to Plesk > domain.com > Mail > user@domain.com > Spam filter > Hits required for spam. Set this value to 3 or 4. The lower the value, the strict the filter.
Login to Plesk > domain.com > Mail > user@domain.com > Spam filter > Training
More info can be found here : http://download1.swsoft.com/Plesk/Plesk8.2/Windows/Docs/plesk-8.2-win-clients-guide/28053.htm