<script> //function to check for a specific email address function donotreply() { if (document.forms["reply"]["send_to"].value == 'do-not-reply@example.com') { alert('You would try to send to the one address yer not allowed to....try again!'); return false; } return true; } </script>
And the form entries…
<form name="reply" onsubmit="return donotreply()"> <input type="text" name="send_to">