Plesk Automation Microupdate installation breaks the greylisting configuration.
After microupdate installation, the file
/var/lib/plesk/mail/greylist/settings.db on mail server is being overwritten with default values.
Meanwhile the greylisting configuration remains unaltered in Plesk Automation. Thus despite the control panel shows the greylisting is
OFF, in fact it is
The behavior is caused by software issue #PPA-3017.
The fix is included in Plesk Automation MU#22.
Install latest PA updates according to the article.