Plesk’s Spamassassin is an OK spamfighter, but it isn’t the best around. At least, not with its default configuration.
You can however, tune it further to include DNS Blacklists (or DNSBL ) to make use of Spamhaus.org’s lists . You can also make it check for SPF records (the so-called Sender Policy Framework ) with its senders, to make it even more strict.
These options can be found in your Plesk controlpanel, under “Server” > “Mail” > “Preferences”.
According to Spamhaus themselves, it’s advised to add the “zen.spamhaus.org” as a DNSBL address (a combination of all their major DNSBL’s). I have yet to play with the SPF-record spam protection – but I’ll make sure to write something extra about that in a couple of days.