Have to pose the question - why Exchange 5.5 rather than Exchange 2K? If they are on Win2K, you'd be better off with Exchange 2K. However, the setup is a bit different (easier), so if you change your mind and decide to do 2K instead of 5.5, then you need to re-post your question.
For Exchange 5.5, you need to do the following:
1. Get a static IP address and have your ISP add a host and an MX record on their DNS servers (assuming they are authoritative for the domain) to resolve that IPto your Exchange server. You can use any name you want - mail.domain.com is a common one.
2. Configure your users with appropriate SMTP addresses, i.e., user@domain.com.
3. From the Exchange Administrator, go to File/New Other/Internet MailServer to install the Internet Mail Connector. I think you will probably need to reinstall the service pack after adding the IMS. You should ideally be on SP4, anyway.
4. Now you need to configure the IMS to connect to your ISP properly. Rather than trying to describe this myself, here's one white paper that might be helpful: "Configuring Internet Mail Service on MS Exchange Server 5.5". You should be able to find this by search the MS TechNet web site.
5. If your Exchange server isbehind a proxy server or firewall, you will need to do some additional steps. First, you need to allow packets on ports 25 and 110 and forward them to your Exchange server's internal IP. Depending upon what proxy or firewall you're using, you may find other things are necessary, but I can't tell you exactly what - I know for MS Proxy there are some registry entries you need to change on your Exchange server.
If you run into problems or have additional questions, feel free to email me at synergy@synoffsys.com.
Hope this helps!
