It is possible to run Outlook 2003 in cached mode, that hits the Exchange
2003 server over HTTP, which effectively does what you want, I think,
although the Server may have to open a port or two...
Try:
http://www.microsoft.com/office/ork/2003/three/ch8/OutC07.htm
You can configure user accounts in Microsoft® Office Outlook® 2003 to
connect to Microsoft Exchange Server 2003 over the Internet without the need
to use virtual private network (VPN) connections. This feature - connecting
to an Exchange account by using Remote Procedure Call (RPC) over HTTP -
allows Outlook users to access their Exchange Server accounts from the
Internet when they are traveling or are working outside their organization's
firewall.
Hope this helps,
Dix