E2K…Send and Receive

Hi Everybody,
Mail boxes for a branch company users are hosted on E2K server located at the main office at another location. After I completed the configuration, users at the branch office were able to send and receive email internally but can’t send or receive exteral mail(the mail be returned back to them)+ they can send and receive to and from the main office. There is no server at the branch site, they use the internet to download email from the main office. For me, it seems there is some kind of connector problem.The other important poit is that the remote users didn’t have e-mail serveice before.
*** What can I do to make the remote site to send and receive external mail?***