Exchange over VPN

I have a problem. I have two linksys routers that are connected together via VPN. Location A can ping Location B. Location B can ping Location A. THey can share and transfer files ok. BUt when it comes to e-mail, no way. Exchange 2000 by the way, From the remote location I cannot use Exchange, yet I can ping and share files ok.

Some of my machines are Win98 and XP home, since these are not able to join domains, coud that be the issue? Thanks in advance.