<add key="SMTPServer" value="mail.test.com"/>
<add key="MailUserName" value="admin@test.com"/>
<add key="MailPassword" value="password"/>

Then use namespace on source code file.

i.e.
using System.Net.Mail;

Now write down the below code on button click (on which you want to send email.)with required perameter. i.e.


SmtpClient objSMTP = new SmtpClient(ConfigurationManager.AppSettings ["SMTPServer"].ToString());

objSMTP.Credentials = new System.Net.NetworkCredential(
ConfigurationManager.AppSettings["MailUserName"].ToString(),

ConfigurationManager.AppSettings["MailPassword"].ToString());

MailMessage objMailMessage = new MailMessage();

objMailMessage.From = new MailAddress(ConfigurationManager.AppSettings["MailUserName"].ToString());           

objMailMessage.Subject = "Test mail";

objMailMessage.To.Add(new MailAddress(EmailAddress));

objMailMessage.Body = "Hi, this is a text mail.

objMailMessage.IsBodyHtml = true;

objSMTP.Send(objMailMessage);

lblMsg.Visible = true;

lblMsg.Text = "Mail has been sent.";