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 license Error loading server unique id"/
Plesk Onyx 17.5.3 on CentOS 7.4 64-bit with latest updates.
Plesk License leased by OVH for their mini SP sever.
I've requested help from support via filling the support form,
and searched this forum for a solution. However, uninstall and re-install,
as was recommended, didn't help.
Looking forward to the reply from support.
Igor V. Timofeev