Adding 127.0.0.1 to Denied from the networks that are not listed option is selected under Tools & Settings > Restrict Administrative Access > Settings
solved the problem. MagicSpam Pro is activated and enabled on the server.

After uninstalling MagicSpam Free Edition and installing MagicSpam Pro for Plesk as advised, via downloading magicspampro-plesk-2.1-8-rhel764.x86_64.zip from http://www.magicspam.com/download, I got the following error trying to enble it by entering the license key: "error fetching plesk licens...