Use the ASP.Net built in class, System.Net.Mail, to send email through your ASP.Net page with SMTP authentication.
<%@ Import Namespace="System.Net" %>
<%@ Import Namespace="System.Net.Mail" %>
<script language="C#" runat="server">
protected void Page_Load(object sender, EventArgs e)
{
MailMessage mail = new MailMessage();
mail.From = new MailAddress ( "postmaster@YourSiteDomain.com" );
mail.Subject = "This is a test message" ;
mail.Body = "If you can see this mail, your SMTP service is working" ;
mail.To.Add( "recipient@somewhere.com" );
SmtpClient smtp = new SmtpClient( "mail.YourSiteDomain.com" );
NetworkCredential credential = new NetworkCredential ("postmaster@YourSiteDomain.com" , "password");
smtp.Credentials = credential;
smtp.Send(mail);
Response.Write( "Message was sent to " + mail.To + " at " + DateTime .Now);
}
</script>