i am sending mail using gmil but is showing such kind of exception pleasse see this code and rectify the exception?

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ASP.NET

please see this code written below this showing the exception like that:

how i can solve this exception:
could not sent the mailThe SMTP server requires a secure connection or the client was not authenticated. The server response was: 5.5.1 Authentication Required. Learn more at

<table border="0" cellpadding="0" cellspacing="0">
            <tr>
                <td style="width: 80px">
                    To:
                </td>
                <td>
                    <asp:TextBox ID="txtTo" runat="server"></asp:TextBox>
                </td>
            </tr>
            <tr>
                <td>
                    &nbsp;
                </td>
            </tr>
            <tr>
                <td>
                    Subject:
                </td>
                <td>
                    <asp:TextBox ID="txtSubject" runat="server"></asp:TextBox>
                </td>
            </tr>
            <tr>
                <td>
                    &nbsp;
                </td>
            </tr>
            <tr>
                <td valign="top">
                    Body:
                </td>
                <td>
                    <asp:TextBox ID="txtBody" runat="server" Height="150" TextMode="MultiLine" Width="200"></asp:TextBox>
                </td>
            </tr>
            <tr>
                <td>
                    &nbsp;
                </td>
            </tr>
            <tr>
                <td>
                    &nbsp;
                </td>
                <td>
                    &nbsp;
                </td>
            </tr>
            <tr>
                <td>
                    &nbsp;
                </td>
            </tr>
            <tr>
                <td>
                    Gmail Emal:
                </td>
                <td>
                    <asp:TextBox ID="txtEmail" runat="server"></asp:TextBox>
                </td>
            </tr>
            <tr>
                <td>
                    &nbsp;
                </td>
            </tr>
            <tr>
                <td>
                    Gmail Password:
                </td>
                <td>
                    <asp:TextBox ID="txtPassword" runat="server" TextMode="Password"></asp:TextBox>
                </td>
            </tr>
            <tr>
                <td>
                    &nbsp;
                </td>
            </tr>
            <tr>
                <td>
                </td>
                <td>
                    <asp:Button ID="Button1" runat="server" OnClick="SendEmail" Text="Send" />
                </td>
            </tr>
        </table>

coding C# asp.net

protected void SendEmail(object sender, EventArgs e)
   {
       try
       {
           SmtpClient smcl = new SmtpClient();
           smcl.Host = "smtp.gmail.com";
           smcl.Port = 587;
           smcl.EnableSsl = true;
           NetworkCredential nc = new NetworkCredential(txtEmail.Text,txtPassword.Text);
           smcl.Credentials = nc;
           smcl.UseDefaultCredentials = false;
           MailMessage mmsg = new MailMessage();
           MailAddress rec = new MailAddress(txtTo.Text);
           MailAddress sen = new MailAddress("navneetvats18@gmail.com");
           mmsg.Subject = txtSubject.Text;
           mmsg.Body=txtBody.Text;
           mmsg.To.Add(rec);

           mmsg.Sender = sen;
           mmsg.From = sen;
           smcl.Send(mmsg);
           Response.Write("Email send");
       }

       catch (Exception ex)
       {
           Response.Write("could not sent the mail" + ex.Message);

       }
   }