Incoming Spam Plesk
Reduce incoming spam
To help reduce the amount of spam that you receive, you should do the following in Plesk 12+.
Click on Tools & Settings, then choose Mail Server Settings.
Enable "Switch on SPF spam protection". In the SPF checking mode section, select "Reject mail when SPF resolves to "fail" (deny)
Enable "Switch on spam protection based on DNS blackhole lists"
In the DNS zones for DNSBL service field, enter the following:
zen.spamhaus.org;b.barracudacentral.org