<?xml version="1.0" encoding="utf-8" ?>
<configuration>

  <appSettings>
    <add key="PFUserName" value="abc@gmail.com"/>
    <add key="PFPassWord" value="xxxxxxxxxxxxx"/>
    <add key="smtpClient" value="smtp.gmail.com"/>
    <add key="SmtpServerPort" value="587"/>
  </appSettings>
</configuration>

using System.Configuration;
namespace SendingMessage.ControlTemplates.SendingMessage
{
    public partial class SendingMessage : UserControl
    {

        protected void btn1_Click(object sender, EventArgs e)
        {
            string userName = ConfigurationManager.AppSettings["PFUserName"];
            string password = ConfigurationManager.AppSettings["PFPassWord"];
            string smtpClient = ConfigurationManager.AppSettings["smtpClient"];
            string smtpPort = ConfigurationManager.AppSettings["SmtpServerPort"];
            try
            {
                MailMessage mail = new MailMessage();
                SmtpClient SmtpServer = new SmtpClient(smtpClient);
                mail.From = new MailAddress("xyz@gmail.com");
                mail.To.Add(lbl_mailId.Text.Trim());
                mail.Subject = "Test Mail";
                mail.Body = txt_mail.Text.Trim();
                SmtpServer.Port = int.Parse(smtpPort);
                SmtpServer.Credentials = new          System.Net.NetworkCredential(userName, password);
                SmtpServer.EnableSsl = true;
                SmtpServer.Send(mail);
                txt_mail.Text = "";
            }
            catch (Exception ex)
            {

            }
        }
    }
}