Emails sent by Sitebuilder Feedback module are deleted as Fraud.
It's by design that emails from Feedback form of Parallels Plesk Sitebuilder 4.5.5 are sent from the publishing host (the server where to your site was published) using standard mail function. Here you may find some default about configuration of the Feedback form:
Feedback form of Web Presence Builder 11 has been improved - many issues that caused email broking by spam protection have been fixed. It's recommended that you install Web Presence Builder 11 Standalone Addition and import your sites from Parallels Plesk Sitebuilder 4.5 using the migraiton tools.