protected void btnSendEmail_Click(object sender, EventArgs e)
{
    // Create Mail Message Object with content that you want to send with mail.
    System.Net.Mail.MailMessage MyMailMessage = new System.Net.Mail.MailMessage("dotnetguts@gmail.com","myfriend@yahoo.com",
"This is the mail subject", "Just wanted to say Hello");

    MyMailMessage.IsBodyHtml = false;

    // Proper Authentication Details need to be passed when sending email from gmail
    System.Net.NetworkCredential mailAuthentication = new
    System.Net.NetworkCredential("dotnetguts@gmail.com", "myPassword");

    // Smtp Mail server of Gmail is "smpt.gmail.com" and it uses port no. 587
    // For different server like yahoo this details changes and you can
    // Get it from respective server.
    System.Net.Mail.SmtpClient mailClient = new System.Net.Mail.SmtpClient("smtp.gmail.com",587);

    // Enable SSL
    mailClient.EnableSsl = true;

    mailClient.UseDefaultCredentials = false;

    mailClient.Credentials = mailAuthentication;

    mailClient.Send(MyMailMessage);
}