in the rt-mailgate man, in the SETUP says:
‘Much of the set up of the mail gateway depends on your MTA and mail routing configuration. However, you will need first of all to create an RT user for the mail gateway and assign it a password; this helps to ensure that mail coming into the web server did originate from the gateway’.

About what user talks ?
Irene Gutiérrez Ruesga
Responsable del área de atención al Usuario
Servicio de Informática de la Universidad de Cantabria
soporte@unican.es Tfn: +34 942-201181