Time-Out Values for Exchange

We switched one of our satellite offices from direct frame into our intranet to a full T1 line for their internet access. Since the move, their connection to our Exchange 5.5 server seems to time-out periodically. This forces them to hit the send/receive sometimes, but not always, to clear the outbox. The hosts file is amended for the Exchange's public address and the firewall does not have a time-out value for connections. What might cause a user to have to resync with the exchange server periodically and force a send/receive?