Hi Will,
I just replied to your ticket few mins ago.. Here are few basic steps for encountering spams :
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 :
Setting Up Spam Filter