Security: From Outlook to Exchange Srvr

Currently we're running Exchange Server 5.5. Packet filtering through a 1-2-1 NAT, and TrendMicro ExchangeServer Scanner are running between the server and the Internet - but - the clients are not always inside the office. Does anyone have any tips or ideas on how to maybe improve this?
Also, we're going to be using a Shiva VPN Solution (VPN Express)...any thoughts on how Outlook 2000 performs with this environment??

THANKS!