Load the popup on just the first time page request only

protected void Page_Load(object sender, EventArgs e)
    {
        if (IsPostBack)
        {
            ClientScript.RegisterHiddenField("isPostBack", "1");
        }
}
<script type="text/javascript" >

        $(document).ready(function() {

            var isPostBackObject = document.getElementById('isPostBack');
            if (isPostBackObject == null) {
                var id = '#dialog';
                //modal: true
                //Get the screen height and width
                var maskHeight = $(document).height();
                var maskWidth = $(window).width();

                //Set heigth and width to mask to fill up the whole screen
                $('#mask').css({ 'width': maskWidth, 'height': maskHeight });

                //transition effect  
                $('#mask').fadeIn(1000);
                $('#mask').fadeTo("slow", 0.9);

                //Get the window height and width
                var winH = $(window).height();
                var winW = $(window).width();

                //Set the popup window to center
                $(id).css('top', winH / 2 - $(id).height() / 2);
                $(id).css('left', winW / 2 - $(id).width() / 2);

                //transition effect
                $(id).fadeIn(2000);

                //if close button is clicked
                $('.window .close').click(function(e) {
                    //Cancel the link behavior
                    e.preventDefault();

                    $('#mask').hide();
                    $('.window').hide();
                });

            }
<style type="text/css">
  #mask {
  position:absolute;
  padding-left:60px;
  padding-top:80px;
  padding-bottom:80px;
  padding-right:50px; /*-- These Padding is for the border of jquery Pop-up --*/
  left:0;
  top:0;
  z-index:9999;
  background-color:#000000;
  display:none;
}  
#boxes .window {
  position:absolute;
  left:0;
  top:0;
  width:330px;
  height:147px;
  display:none;
  z-index:9999;
  padding:20px;
  background-color:#000000;  
  border: 20px solid #79BBFD;  
  border-radius: 10px;
  -webkit-border-radius: 10px;
  -moz-border-radius: 10px;

}
#boxes #dialog {
  padding:10px;
  background:url('images/localOffers-bg.jpg'); 
  width:330px; height:147px; 
  background-repeat:no-repeat; 
  margin-top:20px;
  
}

<div id="boxes">
<div id="mask" >
<div id="dialog" class="window">
<span class="LocalOffersHeading">Find Local Offers                                     
<a href="#"  style="float: right; margin: -45px -45px 0 0;"  class="close"> 

<img src="Images/close_pop.png" alt="CLOSE" /></a> </span> <br /> <br />
Finding local offers has never been easier. Just select your city location from
the drop down and find the most recent local discounts in your area. We've also
made simple to stay in touch with you favorite merchants but signing up for the
email discounts! <br /><br />
<asp:DropDownList ID="ddlj" runat="server"  style="Width:228px;  Height:20px;" 

 OnSelectedIndexChanged="popup_SelectedIndexChanged" >
</asp:DropDownList>
                                     
<asp:ImageButton ID="jqbutton" runat="server" ImageUrl="~/images/bt-go.jpg"

 OnClick="imgBtnGo_Click"
 CausesValidation="false" Style="height: 16px;"  />
                                       
</div>
<!-- Mask to cover the whole screen -->
<%--<div style="width: 1478px; height: 602px; display: none; opacity: 0.8;" id="mask">
</div>--%>
                                </div>
                            </div>

How to send email Asynchronously in ASP.Net using Background Thread

HTML Markup

<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" TextMode = "MultiLine" Height = "150" Width = "200"></asp:TextBox>

        </td>

    </tr>

    <tr>

        <td>

            &nbsp;

        </td>

    </tr>

    <tr>

        <td>

            File Attachment:

        </td>

        <td>

            <asp:FileUpload ID="fuAttachment" runat="server" />

        </td>

    </tr>

    <tr>

        <td>

            &nbsp;

        </td>

    </tr>

    <tr>

        <td>

            Gmail Email:

        </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 Text="Send" OnClick="SendAsyncEmail" runat="server" />

        </td>

    </tr>

</table>

Namespaces

You will need to import the following namespaces

C#

using System.IO;

using System.Net;

using System.Net.Mail;

using System.Threading;

VB.Net

Imports System.IO

Imports System.Net

Imports System.Net.Mail

Imports System.Threading

Sending email asynchronously in background using Thread

C#

protected void SendAsyncEmail(object sender, EventArgs e)

{

    string to = txtTo.Text;

    string from = txtEmail.Text;

    string password = txtPassword.Text;

    string subject = txtSubject.Text;

    string body = txtBody.Text;

    HttpPostedFile postedFile = fuAttachment.PostedFile;

    Thread email = new Thread(delegate()

    {

        SendEmail(to, from, password, subject, body, postedFile);

    });

 

    email.IsBackground = true;

    email.Start();

    ClientScript.RegisterStartupScript(GetType(), "alert", "alert('Email sent.');", true);

}

 

private void SendEmail(string to, string from, string password, string subject, string body, HttpPostedFile postedFile)

{

    using (MailMessage mm = new MailMessage(from, to))

    {

        mm.Subject = subject;

        mm.Body = body;

        if (postedFile.ContentLength > 0)

        {

            string fileName = Path.GetFileName(postedFile.FileName);

            mm.Attachments.Add(new Attachment(postedFile.InputStream, fileName));

        }

        mm.IsBodyHtml = false;

        SmtpClient smtp = new SmtpClient();

        smtp.Host = "smtp.gmail.com";

        smtp.EnableSsl = true;

        NetworkCredential NetworkCred = new NetworkCredential(from, password);

        smtp.UseDefaultCredentials = true;

        smtp.Credentials = NetworkCred;

        smtp.Port = 587;

        smtp.Send(mm);

    }

}

VB.Net

Protected Sub SendAsyncEmail(sender As Object, e As EventArgs)

    Dim [to] As String = txtTo.Text

    Dim from As String = txtEmail.Text

    Dim password As String = txtPassword.Text

    Dim subject As String = txtSubject.Text

    Dim body As String = txtBody.Text

    Dim postedFile As HttpPostedFile = fuAttachment.PostedFile

    Dim email As New Thread(Sub() SendEmail([to], from, password, subject, body, postedFile))

    email.IsBackground = True

    email.Start()

    ClientScript.RegisterStartupScript([GetType](), "alert", "alert('Email sent.');", True)

End Sub

 

Private Sub SendEmail([to] As String, from As String, password As String, subject As String, body As String, postedFile As HttpPostedFile)

    Using mm As New MailMessage(from, [to])

        mm.Subject = subject

        mm.Body = body

        If postedFile.ContentLength > 0 Then

            Dim fileName As String = Path.GetFileName(postedFile.FileName)

            mm.Attachments.Add(New Attachment(postedFile.InputStream, fileName))

        End If

        mm.IsBodyHtml = False

        Dim smtp As New SmtpClient()

        smtp.Host = "smtp.gmail.com"

        smtp.EnableSsl = True

        Dim NetworkCred As New NetworkCredential(from, password)

        smtp.UseDefaultCredentials = True

        smtp.Credentials = NetworkCred

        smtp.Port = 587

        smtp.Send(mm)

    End Using

End Sub

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