Define a rule to delete all messages received through account x@x.com
that are not known senders (i.e., in a contact-type folder).

Apply this rule after the message arrives
through the <account>
delete it (or whatever action you want)
except if sender is in <contactfolder> Address Book
[stop processing more rules]

The last clause is optional.  If you don't want further rules to get
exercised against the same message then don't let Outlook proceed
further.