Can’t enable DNSBL/RBL in Plesk because it’s greyed out
If you have a new Plesk installation and the following option is greyed out in Server -> Mail:
Switch on spam protection based on DNS blackhole lists
Just install the following RPM from Plesk:
psa-qmail-rblsmtpd
This is called ‘SPAM blocker for QMail daemon’ within the updater in the Plesk interface.