DNS blacklist is very useful to limit incoming spam coming to a server.
It is easy to enable DNS blacklist server.
Please use below steps to DNS blacklist in a plesk server.
1. Login to Plesk as admin
2. Go to Tools & Settings
3. Go to Mail >> Mail Server Settings
4. Scroll down to "Switch on spam protection based on DNS blackhole lists".
5. Tick "Switch on spam protection based on DNS blackhole lists".
6. Fill following in the box near to "DNS zones for DNSBL service"
7. Press OK