Forward all mails to other server, how?

Hi,

probably I'll integrate a new mail gateway for antivirus and antispan filtering soon. This gateway haven't smtp-auth by itself, so, I have thinked this for integrate it into my network:

GW = MailGateway
MS = MailServer (qmail-ldap)

From external servers, all connections arrive to GW: 192.168.1.1:25. GW accept mail if it's a mail for my domain, filter an redirect to MS: 192.168.1.2:X. MS delivery the messages.

From my clients, all connections arrive to MS:192.168.1.2:25 or 587 port directly. MS should forward all mails to GW: 192.168.1.1:25. GW filter and redirect to MS: 192.168.1.2:X and finally, MS delivery the mail to internal or external user.

The problem is: How can I forward incoming MS mail to GW?

Thanks!