Well, I'd do this with server-side software rather than an
applet. Sending mail from an applet is usually more trouble than it's worth.
If your server runs on a Unix or Linux system, a little CGI script which echoes a message including the $HTTP_REFERER url to "mail" with a mail id entered in a form textfield should do the trick. You may even be able to find a pre-written one in one of the many script archives on the web.
Or if you are feeling keen you could write a
Java servlet to do the same job.