VPN…

When user try to reply any msg. through at & t dialup connection and office 2000 outlook they get the following error ” No transport provider was available for delivery to this recipient” but composing new mail works fine. We are facing this problem when user goes out and try to connect through vpn to our exchange 5.5 mail server. We are running vpn on checkpoint firewall.
Please advise me about this problem..
Thanks in advance..
chandan.