smtp mail and closed relay

i’m a new admin and my company has 3 locations our main location has an exchange server the other 2 locations conect to it via pop&smtp the old admin had an open relay ( not good ) however the other locations could send out mail to anyone and now that i’v closed the relay that are only able to e-mail within the company. now one of our locations is comming in via ip address and the other is comming in via VPN the location comming in via ip I set them up so the smtp is going to there ISP and it works fine. here comes the question the one that comes in via the VPN can not get to the isp’s smtp address i’v tryed to get the connected via the exchange plugin but it hanges. is there a way to open the relay to there proxy server?