A case should be created without any exception criteria. I have configured below condition for Automatic Case creation and Update rules
View Properties:
Case is created for all incoming emails. if Contact does not exists in CRM as a User/Contact then Contact is also getting created. If any Originator sends an email putting some other people in To/CC
(To/CC does not exists as a Contact in CRM) then Case is also created and Contact is appended as customer on Case. but if any one in TO/CC replies then the same Case is updated with the Customer also. Customer should not get updated if any update is sent on the existing Case.