Add GUI for email greylisting per email account in Plesk Onyx. Also add a 'Detection Only' mode to allow a testing period, similar to spam filters and mod_security.
Thank you for your input! We will consider this functionality in upcoming releases if it will be popular.
Everyone, please continue voting for this feature if you consider it important.
For Plesk 17 the solution may be http://www.haggybear.de/de/greylisting-manager but for Plesk 18 it doesn't work. It would be good to have a solution from Plesk directly.