Contact Form with user id instead of email address?

Hello again,
I’m trying to build a contact form and instead of entering email address, i want the user to enter its user id and I will include the @domain.com on actionscript on the form and on its php.
Something like this:
User id:[COLOR=#0000ff]"________"[/COLOR] ([COLOR=#0000ff]enter by user[/COLOR]) @ domain.com (@domain.com will be already on the form)
so all the user has to do is enter its user id.
this user id will pick up the rest which will be the @domain.com and that would be an email send.
Has anyone done that before?.. any help please!


on (release) {
 
 if (!Email.length || Email.indexOf("@") == -1 || Email.indexOf(".") == -1) {
  EmailStatus = "Please enter a valid COMPANY E-mail address";
 }   
 else if (!ToComments.length) {
  EmailStatus = "Please enter your message";
 } 
 
 else {
  loadVariablesNum ("MailPHP.php", "0", "Post");
        gotoAndPlay(10);
 }
}

and this is what i have on my php file


<?
$ToEmail = "email@domain.com";
$ToSubject = "Training Orientation Feedback";
$EmailBody = "Sent By: $FirstName
Senders Email: $Email

Message Sent:
$ToComments
";
mail($ToName." <".$ToEmail.">",$ToSubject, $EmailBody, "From: ".$FirstName." <".$Email.">");
?>

my input text variable is called Email

thanks for the help.