Dependiendo del origen y de la variable $gis_system saldrá por bookings@golfinspain.com o system@golfinspain.com:

if($origen == 'bookings@golfinspain.com' || $swift_host_from == 'golfinspain.com'){
    $swift_server = 'smtp.gmail.com';
    $swift_port = 465;
    $swift_method = 'ssl'; // si no se quiere poner a null
    if($gis_system == 1 || $_REQUEST['system'] == 1){
        $swift_username = 'system@golfinspain.com';
        $swift_password = 'greendel182015';
    } else {
        $swift_username = 'bookings@golfinspain.com';
        $swift_password = 'greendel18';
    }
} elseif($swift_host_from == 'golftaste.com'){
    // amazon/mailgun
    $swift_server = 'smtp.gmail.com';
    $swift_port = 465;
    $swift_method = 'ssl'; // si no se quiere poner a null
    $swift_username = 'bookings@golfinspain.com';
    $swift_password = 'greendel18';
} else {
    $defaultSendmail = true;
}