You cannot connect to Microsoft Exchange through the Internet by using remote procedure call (RPC) over Hypertext Transfer Protocol (HTTP).
This problem occurs because Outlook does not correctly detect the settings in the Local Area Network (LAN) Settings dialog box when you connect to Microsoft Exchange through the Internet by using RPC over HTTP if both of the following conditions are true:
You are using an automatic configuration script.
You have not specified the proxy server or the port number in the Local Area Network (LAN) Settings dialog box.
Caution Before changing any settings so that you can use HTTP to connect to your Microsoft Exchange Server mailbox, contact your Exchange server administrator.
To resolve this problem, specify both the proxy server and the port number in the Local Area Network (LAN) Settings dialog box in Microsoft Internet Explorer. To do this, follow these steps.
Note Contact your Exchange server administrator to obtain the information to complete these steps. For example, you must know the automatic configuration script and the correct name of your proxy server and port number.
Start Internet Explorer, and then click Internet Options on the Tools menu.
Click the Connections tab, and then click LAN Settings to open the Local Area Network (LAN) Settings dialog box.
Under Automatic Configuration, click to select the Use Automatic Configuration Script check box. Type a valid script.
Click to select the Use a proxy server for your LAN check box.
In the Proxy Server area, type the name of your proxy server in the Address box, and then type the port number in the Port box.
In a local area network (LAN) environment, Microsoft Outlook communicates with the Microsoft Exchange server by using remote RPC with TCP/IP. This method provides quick, efficient access for a corporate network. However, in previous versions of both Exchange server and Outlook, access to an Exchange server by remote users required a virtual private network (VPN) connection to the corporate network. A VPN provides a remote user with a connection in an organization's corporate network and within its firewall. A VPN enables access to more network services than is required for just e-mail access.
For remote connections, Outlook now offers an alternative to VPN connections. Outlook can connect to an Exchange server through the Internet by using RPC over HTTP. This feature allows you to remotely access your Exchange server account from the Internet when you are working outside your organization’s firewall without any special connections or hardware, such as smart cards and security tokens.
There are several minimum requirements to use this feature. These include the following requirements:
Microsoft Windows XP Service Pack 1 (SP1) with Microsoft Knowledge Base article 331320 updates installed, or a later Service Pack. Microsoft Exchange Server 2003 running on Microsoft Windows Server 2003.
Your Exchange server administrator must have configured the server to permit connections through HTTP. For more information about bow to configure this feature, see the Microsoft Office Resource Kit and the Microsoft Exchange Server documentation.