Our main Exchange 2003 server is at a hosting provider. We have a site-to-site VPN connection to them but we always access the mail server through the Internet, not the VPN. I recently added an Exchange 2003 backend server plus an Exchange 2003 frontend server at our local office in preparation to move all email locally. This would be a gradual transition where mailboxes would be moved over a few at a time. I've done testing on this setup and all servers can communicate with each other fine through the VPN; all mail is delivered to the appropriate mailboxes. I moved my mailbox to the new local server and used the new frontend server for a week without incident. When I went live with this new configuration, I made the change in internal and external DNS to point all users to the new frontend server. Things were fine for a day or two but then the VPN hardware on our side became nonresponsive. Turns out it could not handle the load.
My question is is there a way for the local and hosted Exchange servers to communicate with each other over the Internet instead of the VPN route? Every search I've done on this topic involves using a VPN connection. I'm looking for a definite yes or no on whether this can be done, and if it can what are the steps to do it.