An exception occurred during a WebClient request.

Customized cultures cannot be passed by LCID, only by name.

public static void Send(string message)
{
    var url = "https://api.mailgun.net/v3/sandbox1ebb54bdb39044988d3c916d833769b7.mailgun.org/messages";
    using (var client = new WebClient())
    {
        client.Credentials = new NetworkCredential("api", "some_password");
        var values = new NameValueCollection();
        values["from"] = "Mailgun Sandbox <postmaster@sandbox1ebb54bdb39044988d3c916d833769b7.mailgun.org>";
        var response = client.UploadValues(url, values);
        var responseString = Encoding.Default.GetString(response);
        Reader.log(responseString);
    }
}