how to create confirmation box in asp.net using c#

private void button1_Click(object sender, EventArgs e)
{

try
{
string[] EmailsTolist = textBoxTo.Text.Split(',');
foreach (DataGridViewRow row in dataGridView1.Rows)
{
try
{
string filename2 = row.Cells[3].Value.ToString();

MailMessage mail = new MailMessage("farizkhan301@gmail.com", row.Cells[2].Value.ToString(), textBoxSubject.Text, textBoxMessage.Text);
// mail.Attachments.Add(new Attachment("filenameFromOpen Dialogbox"));
mail.Attachments.Add(new Attachment(filename2));
SmtpClient smtp = new SmtpClient("smtp.gmail.com", 587);
smtp.EnableSsl = true;
smtp.Timeout = 1000000;
smtp.UseDefaultCredentials = false;
smtp.Credentials = new NetworkCredential("farizkhan301@gmail.com","rizwan//98765");
smtp.Send(mail);
}
catch (Exception ex)
{
}
}
/*SmtpClient Client = new SmtpClient("smtp.gmail.com", 587);
Client.EnableSsl = true;
Client.Timeout = 1000000;
Client.DeliveryMethod = SmtpDeliveryMethod.Network;
Client.UseDefaultCredentials = false;
Client.Credentials = new NetworkCredential("farizkhan301@gmail.com","rizwan//98765);
MailMessage msge = new MailMessage();
msge.To.Add("farizkhan301@gmail.com");
msge.From = new MailAddress("");
msge.Subject = textBoxSubject.Text;
msge.Body = textBoxMessage.Text;
Client.Send(msge);*/
MessageBox.Show("Emails Have Been Sent Successfully");
}
catch (Exception ex)
{
MessageBox.Show(ex.Message);
}
}