Checking Email

In this example $_GET['cntnt01fbrp__39'] is an ajax call from formbuilder... You would/could change the get to email or something. This also suggest some valid alternates... ie: if you are not using an email for the user name then you would want to give the alternates...

global $gCms;
  $FEU = $gCms->modules['FrontEndUsers']['object'];

// A FUNCTION TO TEST FOR A VALID EMAIL ADDRESS, RETURN TRUE OR FALSE 
function check_valid_email($email){ 
    // IS THE PATTERN OF THE EMAIL ADDRESS OK? 
    if (!preg_match('/^[A-Z0-9_-][A-Z0-9._-]*@([A-Z0-9][A-Z0-9-]*\.)+[A-Z]{2,6}$/i', $email)) return FALSE; 
    // IS THE DOMAIN OF THE EMAIL ADDRESS ROUTABLE OVER THE INTERNET FOR MX OR A RECORDS? 
    $emaila = explode('@', $email); 
    if ( checkdnsrr($emaila[1],"MX") || checkdnsrr($emaila[1],"A") ) return TRUE; 
    // NOT ROUTABLE 
    return FALSE; 
} 
if (!empty($_GET['cntnt01fbrp__39'])){ 
    $e = $_GET['cntnt01fbrp__39']; 
    if (check_valid_email($e)){ 
        echo "<br/>VALID: $e \n"; 
        if($FEU->GetUserID($e)){
            $e1=$e;
            while ($FEU->GetUserID($e1)) {
                $e1=$e.rand(5, 100);
            }
            $e2=$e.rand(5, 100);
            while ($FEU->GetUserID($e2)) {
                $e2=$e.rand(5, 100);
            }
            echo '<span style="color:red">This is not available</span>
            <span style="color:green">'.$e1.' and '.$e2.' is available</span>';
            return 'false';
        }else{
            echo "<br/>AVAILABLE: $e \n"; 
            return 'true';
        }
    }else{ 
        echo "<br/>BOGUS: $e \n"; 
        return 'false';
    } 
}