smart_route:
     driver = manualroute
     domains = ! +local_domains
     ignore_target_hosts = 127.0.0.0/8
     condition = "${perl{check_limits}}"

     #use remote_smtp only if you do not need smtp-auth.
     #transport = remote_smtp

     #use auth_relay if you do need to set the remote smtpauth
     transport = auth_relay

     route_list = * smtp.yourisp.com
     no_more