Here’s the dilly…
1. You do not need ISA server. Microsoft ISA is a software firewall/nat and has no effect on Exchange. You can filter incoming traffic to Exchange but it has no bearing on how Exchange operates.
2. LCS or Live Communications Server 2003/2005 (I suggest 2005) integrates with Exchange 2003 and runs using the SIP protocol. It uses MSN Messenger 5.x that comes with LCS. You can also add your .NET contacts to the client.
*** NOTE: Because LCS runs using SIP you cannot download and install the newer Messenger clients from Microsoft because they do not support SIP.
You can open TCP 5060 and UDP 5061 on your firewall and use the 5.x client from the internet just as you would from you LAN.
You can also configure “RPC over HTTP” on your Exchange and enable SSL with a certificate. This will allow your Outlook to use it’s full client over the internet wothout the use of a VPN connection.
Hope this helps…