emailaddress3 and case creation (mail to case)

Hi

We have set up the case module to not convert emails to case for unknown senders. (we dont wont the system to create new contacts)

The problem is that the system does not seem to connect emailaddress3 to a contact. If we fill in the emailaddress3 field with a email address the system consider this to be a unknown sender and will not convert to case. Is it possible to make some kind of relationship or something to connect this so the system recognize this as the contacts emailaddress when the case creation rule triggers? 

Thanks in advance!