I won't install something like Mailscanner on a system that is management by an external tool like Plesk. Both Mailscanner and Plesk change a lot of things and expect certain conditions.
Mailscanner brings a lot of packages and is not that easy to set up, btw. You really have to know what you're doing. If you're using the Ubuntu package, it's a little bit after the current Mailscanner stable release - but if you install from source, you won't get that warm and comforting feeling of your Plesk setup (Install it on another machine and you'll know what I mean).
Maybe you're better of if you add some simple restrictions to your
smtpd_recipient_restrictions line (main.cf of postfix, if you use it). See the Spamhaus ZEN blocklist and a Greylisting server - these can be added really fast without breaking anything and stop a huge amount of spam.