Firewall problems
So far you've verified that you've got a good communications link and that the DNS server is doing its job. If your clients are still unable to connect to the OWA server, it's probably either due to a firewall problem or an IIS failure. Testing for a firewall problem is easy. Just try to access the OWA server from behind your firewall. If both the OWA server and the client machine exist on your private network behind the same firewall, you can use the client machine to test the OWA server without the firewall interfering. If the test is successful, then the firewall is your problem. Verify that ports 80 and 443 are open to inbound traffic. IIS problems
If the firewall doesn't seem to be the source of your problems, then there's a really good chance that IIS is causing the problem. To test IIS, begin by selecting the Programs | Administrative Tools | Services commands from the Start menu. When you do, Windows will open the Service Control Manager. Go through the Service Control Manager and verify that the following services are running:

• World Wide Web Publishing Service
• IIS Admin Service
• Protected Storage
• Remote Procedure Call (RPC)

This is also a good time to verify that the various Microsoft Exchange-related services are running as well. Once you've verified that the necessary IIS services are running, open Internet Explorer directly on the OWA Server and enter the OWA Server's IP address into the browser. If the OWA session starts, IIS is working correctly. If you can't get OWA to start by entering the IP address, verify that IIS is configured to use the correct IP address. To verify that IIS is configured to use the correct IP address, select Internet Services Manager from the Administrative Tools menu. When the Internet Information Server console opens, select the OWA Web site from the console tree. Before continuing, verify that the word Stopped doesn't appear in parenthesis next to the Web site name. If it does, simply right-click the site and select the Start command. If you receive an error message, your OWA site probably has an IP address conflict with another site on the server. To solve this problem, read the following instructions for verifying the IP address, and then try to start the site again. Verifying the OWA site's IP address
To verify the site's IP address, right-click the OWA site and select Properties. On the site's property sheet, select the Web Site tab and verify that the IP address is correct. By default, the IP address will be set to All Unassigned. However, the All Unassigned setting should be used only for the default Web site. The OWA site should have a dedicated IP address. While on the Web Site tab, you should also verify that the OWA site is configured to use port 80. If all of your settings are correct but you still can't access the OWA Web site, the best thing to do is to implement a sample Web site to verify IIS's functionality. To do so, open the Control Panel and double-click the Network And Dial Up Connections icon. When the Network And Dial Up Connections window opens, right-click your main network connection and select Properties. On the connection's property sheet, select the TCP/IP protocol from the list and click the Properties button to reveal the TCP/IP property sheet. On the TCP/IP property sheet, click Advanced to reveal the Advanced TCP/IP Settings property sheet. On the IP Settings tab of the Advanced TCP/IP Settings property sheet, click the Add button under the IP Addresses section and add a unique IP address to the server. Click OK on all of the open windows to close them. Now, create a directory called Test on your hard disk and place a few random HTML files into it. Be sure to name one of the files INDEX.HTM. At this point, return to the Internet Services Manager. Right-click the server name in the console tree and select New | Web Site. This will launch the Web site creation wizard.