HTTP POST Returns Error: 417 "Expectation Failed."

Another way -

Add these lines to your application config file configuration section:

<system.net>
    <settings>
        <servicePointManager expect100Continue="false" />
    </settings>
</system.net>

System.Net.HttpWebRequest adds the header 'HTTP header "Expect: 100-Continue"' to every request unless you explicitly ask it not to by setting this static property to false:

System.Net.ServicePointManager.Expect100Continue = false;

Some servers choke on that header and send back the 417 error you're seeing.

Give that a shot.