rule #1 when working with forms:
The Form data can only be processed on the forms action page.
the page you have sent is named "admin_Users_Results_Email.php". the action of the form with the userid checkbox is to post to a different page named "admin_Users_Mass_Email.php":
<form action="admin_Users_Mass_Email.php" method="post" id="usermassemail" name="usermassemail">
when the form is submitted, the action page is loaded to receive the form data. you can only validate the form data on that page.
you will either need to move the validation code to the "admin_Users_Mass_Email.php" page, or change the action of the form to post to the "admin_Users_Results_Email.php" page instead.