Greylisting Manager Modul from http://www.haggybear.de/
Since the extension you mention is already available, what exactly is the reason to include this functionality directly into Plesk? What greylisting features are missing from Plesk greylisting?
UPD: Unfortunately, we are closing this request as no information was provided over 2 months.
Ivo Birrer commented
I am using this tool too and would appreciated it if something similar would find its way into Plesk,