There are 2 steps you can take to keep spam at a minimum.
First of all you need to install an additional Plesk Module Click Server > Updater You will see a list of Plesk Versions here, update your plesk to the latest version 8.x Tick Real-time Blackhole List client for qmail, choose the install icon on top. You will get an email of the succesfull install.
Go back to the desktop and click Server > Mail
1) Force SPF Checking of incoming mail.
Tick the box that says: Switch on SPF Spam protection
In the dropdown select Reject Mails when SPF resolves to fail. SPF Local rules keep on the default
2) Switch on spam protection based on DNS blackhole lists
In the box enter: sbl.spamhaus.org;zen.spamhaus.org