validating mail in PHP

I found this gem in my backups. It is to validate mails:

= 0)
    {
        if(filter_var($email, FILTER_VALIDATE_EMAIL) === FALSE) return FALSE;
    }

    // IF LOWER-LEVEL PHP, WE CAN CONSTRUCT A REGULAR EXPRESSION
    else
    {
        $regex
        = '/'                        // START REGEX DELIMITER
        . '^'                        // START STRING
        . '[A-Z0-9_-]'               // AN EMAIL - SOME CHARACTER(S)
        . '[A-Z0-9._-]*'             // AN EMAIL - SOME CHARACTER(S) PERMITS DOT
        . '@'                        // A SINGLE AT-SIGN
        . '([A-Z0-9][A-Z0-9-]*\.)+'  // A DOMAIN NAME PERMITS DOT, ENDS DOT
        . '[A-Z\.]'                  // A TOP-LEVEL DOMAIN PERMITS DOT
        . '{2,6}'                    // TLD LENGTH >= 2 AND =< 6
        . '$'                        // ENDOF STRING
        . '/'                        // ENDOF REGEX DELIMITER
        . 'i'                        // CASE INSENSITIVE
        ;
        // TEST THE STRING FORMAT
        if (!preg_match($regex, $email)) return FALSE;
    }

    // FILTER_VAR OR PREG_MATCH DOES NOT TEST IF THE DOMAIN IS ROUTABLE
    $domain = explode('@', $email);

    // MAN PAGE: http://php.net/manual/en/function.checkdnsrr.php
    if ( checkdnsrr($domain[1],"MX") || checkdnsrr($domain[1],"A") ) return TRUE;

    // EMAIL IS NOT ROUTABLE
    return FALSE;
}

// DEMONSTRATE THE FUNCTION IN ACTION
$e = NULL;
if (!empty($_GET["e"]))
{
    $e = $_GET["e"];
    if (check_valid_email($e))
    {
        echo "
VALID: $e \n";
    }
    else
    {
        echo "
BOGUS: $e \n";
    }
}

// END OF PROCESSING - CREATE THE FORM USING HEREDOC NOTATION
$form = <<
TEST A STRING FOR A VALID EMAIL ADDRESS:



ENDFORM;

echo $form;