Send Email from a Domian

So Im stuck with this problem. I'm sending email form my domain which is hosted in Hostgator. I dont really know how it works with hostgator So whenever I send the email from within my domain I get this error:

But as far as my c# code is concerned here it is:

            MailMessage mail = new MailMessage ();
            mail.From = new System.Net.Mail.MailAddress ("sample@gmail.com");
            SmtpClient smtp = new SmtpClient ();
            smtp.Port = 465;  
            smtp.EnableSsl = true;
            smtp.DeliveryMethod = SmtpDeliveryMethod.Network; 
            smtp.UseDefaultCredentials = false;
            smtp.Credentials = new NetworkCredential ("sample@gmail.com", "sample123!"); 
            smtp.Host = "smtp.gmail.com";
            mail.To.Add (new MailAddress ("sample2@gmail.com"));
            string emailName= Request["txtname"];
            string emailEmail= Request["txtemail"];
            string emailSubject= Request["txtsubject"];
            string emailMessage = Request ["txtmessage"];
            mail.IsBodyHtml = true;
            mail.Subject = emailSubject;
            mail.Body = "<b>FROM:</b>" + " " + emailName + " " + emailEmail + "<br /><br />" + "<b>MESSAGE: </b>" + emailMessage;
            smtp.Send (mail);
            //this will send signal to the js in the front end to match the result as "ok" or "success"
            Response.Write("OK");

And my web.config file I have this:

<httpRuntime maxRequestLength="68157440" requestLengthDiskThreshold="68157440" maxUrlLength="1024" />
              </system.web>
                <system.webServer>
                    <security>
                        <requestFiltering>
                            <requestLimits maxAllowedContentLength="68157440" />
                        </requestFiltering>
                    </security>
                    <rewrite>
                        <rules>
                            <rule name="CanonicalHostNameRule1">
                                <match url="(.*)" />
                                <conditions>
                                    <add input="{HTTP_HOST}" pattern="^www\.mydomainsampe\.com$" negate="true" />
                                </conditions>
                                <action type="Redirect" url="http://www.mydomainsample.com/{R:1}" />
                            </rule>
                        </rules>
                    </rewrite>
                </system.webServer>

In js its just the simple call.. So what am I doing wrong ? Has anyone dealt with hostgator hosted domain or is there something I am missing.

Thanks in advance!