Failure sending mail in asp.net

System.Net.Mail.SmtpException: Failure sending mail. ---> System.Net.WebException: The remote name could not be resolved: 'smtp.google.com' at System.Net.ServicePoint.GetConnection(PooledStream PooledStream, Object owner, Boolean async, IPAddress& address, Socket& abortSocket, Socket& abortSocket6, Int32 timeout) at System.Net.PooledStream.Activate(Object owningObject, Boolean async, Int32 timeout, GeneralAsyncDelegate asyncCallback) at System.Net.PooledStream.Activate(Object owningObject, GeneralAsyncDelegate asyncCallback) at System.Net.ConnectionPool.GetConnection(Object owningObject, GeneralAsyncDelegate asyncCallback, Int32 creationTimeout) at System.Net.Mail.SmtpConnection.GetConnection(ServicePoint servicePoint) at System.Net.Mail.SmtpTransport.GetConnection(ServicePoint servicePoint) at System.Net.Mail.SmtpClient.GetConnection() at System.Net.Mail.SmtpClient.Send(MailMessage message) --- End of inner exception stack trace --- at System.Net.Mail.SmtpClient.Send(MailMessage message) at MS_ContactUS.btnSend_Click(Object sender, EventArgs e) in c:\inetpub\wwwroot\MediaSign\MS-ContactUS.aspx.cs:line 48

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Net;
using System.Net.Mail;
using System.Configuration;

public void btnSend_Click(object sender, EventArgs e)
    {
        MailMessage mm = new MailMessage("sender@gmail.com", "receiver@gmail.com");
        mm.Subject = txtSubject.Text;
        mm.Body = "<br /><br />Email: " + txtFrom.Text + "<br />" + txtBody.Text;

        mm.IsBodyHtml = false;

        SmtpClient smtp = new SmtpClient();

        smtp.Host = "smtp.google.com";

        System.Net.NetworkCredential NetworkCred = new System.Net.NetworkCredential();

        NetworkCred.UserName = "sender@gmail.com";

        NetworkCred.Password = "xxxxx";

        smtp.UseDefaultCredentials = true;

        smtp.Credentials = NetworkCred;

        smtp.EnableSsl = true;

        smtp.DeliveryMethod = SmtpDeliveryMethod.Network;

        smtp.Port = 578;
        try
        {
            smtp.Send(mm);
        }
        catch(Exception ex)
        {
            Label1.Text = ex.ToString();
        }
}

<div>
         
        Message from: <asp:TextBox ID="txtFrom" runat="server" /><br />
        Subject: <asp:TextBox ID="txtSubject" runat="server" /><br />
        Message Body:<br />
        <asp:TextBox ID="txtBody" runat="server" Height="171px" TextMode="MultiLine" Width="270px" /><br />
        <asp:Button CssClass="CTD-button" runat="server" ID="btnSend" Text="send" OnClick="btnSend_Click" /> 
        <asp:Label ID="Label1" runat="server" Text="Label"><br /><br />
           
    </div>