Search for purchase orders with no incoming messages

Hi Everyone,

I am having issues trying to create a search that will find purchase orders with the first email in the communications tab being outgoing and then checking to see if the vendor sent an email back. This search is supposed to show purchase orders awaiting that have not been responded to yet in a certain time period. I cannot seem to think of a way to show just purchase orders with no incoming messages. Any ideas?

Thank you,
Zack