How to route BOUNCE mails to real MX instead of local?

Hi,
Im using ISPCONFIG as one of 3 SMTP servers to send out mails.
MX server is totally different , so recieving goes to different server.
When my SMTP sending bouces , it should send bounce message to the outside MX server, however it keeps bounce on local SMTP server. Is there any way i could enable sendnig bounces to external MX ? Like mark that MX is "remote" not local ? (Can be for bounce only email or whole domain)

Thanks