Hosted Exchange in different Countries

I am assisting one of my users in sending/receiving emails to an outside domain. The outside address is using a hosted Microsoft solution. I am blocking most countries through my Firewall and it seems that the message will not pass the firewall inbound. (If i disable the country filtering, the messages will send and receive) I have asked the outside IT what country is hosting their exchange but they cannot tell me. Any ides on what I can do to identify the originating country?

Thanks!