Conflicting SMTP Server

I downloaded E-Mail Server and I am at the Services control panel screen hitting the "Start Service" button and I keep getting a message: [Fatal Error] This Smtp server could not start listening for connections on your specified SMTP port. Please check that no other SMTP or POP3 server is running at the same time with this one. Pay attention to Microsoft SMTP server that comes in the same package with IIS.
What should I do to fix this ?

There is actually some other SMTP server running.

What happens is, both SMTP servers must bind to port 25 in order to receive email messages from the Internet as dictated by the standards. If our E-Mail Server does it first, you will not see a problem with it, however, in case that any other server binds on port 25, our server will not be able to bind to it and will return an error. You need to determine which program is taking port 25 and shut it down or disable it. In order to find it easily, once you get this error, you can issue the netstat -ab > results.txt command on the command line like this:

If you are running Windows Vista and you didn't disable the nagging UAC feature, you will get the "The requested operation requires elevation." response from Vista. If that is your case, you can easily avoid it, by giving it necessary rights. Go to Start -> All Programs -> Accessories -> Command Prompt. Right click on it to display a context menu and choose: "Run As Administrator". Now you can do the above, but this time you will have the necessary permissions.

When you examine a results.txt file later, look for your port 25 (smtp) entries and below it will tell you what other applications are using this port. Here is an example: