Exchange Server and firewall

An Exchange Mail user who has a problem establishing a connection to the server may have one or more TCP ports blocked. The blockage may be from computer or router firewall, or your Internet provider may simply blocks the ports needed by the Exchange Server such as TCP port 135. Various Internet providers, including Cox, Comcast and Verizon have blocked the port 135 at one point and they may still do.

To determine whether the TCP port is blocked, you may simply use 'telnet' command to test out the port. For example, if your server IP is and the port in question is 135...

C:\>telnet 135

Connecting to not open connection to the host, on port 135: Connect failed

The above message indicates that the port 135 is blocked.

Port 135 is used by Outlook to make initial contact with an Exchange Server, and if it cannot establish a connection it will return "Server Unavailable" error message. If this port is blocked, you cannot connect to the server using the traditional connection method. There are two alternative ways to solve this problem.

1. RPC over HTTP

If you can't use port 135, the best workaround is to use Outlook 2003's new feature, RPC-over-HTTP. RPC over HTTP (or HTTPS) uses TCP ports 80 (or 443) to make a connection to the server. Since no Internet Provider blocks ports 80 (http) and 443 (https), this option will always work. To setup RPC over HTTP, please follow the instruction provided by Microsoft or

2. VPN

A second alternative is to use a VPN. A VPN setup requires a VPN server and a client, and it's a bit more complex to setup than the option 1 (RPC-over-HTTP). A VPN establishes a connection using TCP port 1723, and requires your firewall or router support IP protocol 47 (GRE).


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