Hello,
You can set up Preferences for Handling Mail to Nonexistent Users (Mail Bounce) for all the domains at once. To do this, log in to the Parallels Plesk Panel, go Domains, select domains you wish to set a rule on or tick the title bar checkbox if you wish to modify all domains, click on Group Operations, scroll down to the Preferences area and choose what to do with the mail to nonexistent user.
Regards,
Denis.