Qmail and NAT

[Federico]

The problem is:   I've a public Ip address that is the MX record of my domain.   the port 25 of this public ip is natted to the port 25 of a private address (192.168.5.2), on this server I've qmail of course. After that I've a Microsoft Exchange Server (192.168.5.4) that holds all the mail.   I want to make my qmail like a "mail router"   now the configuration of my qmail i quite correct, tha mail outbound is delivered, the problem is in inbound mail.   I've wrote .mydomain.dm:192.168.5.4 in smtproutes   I note that when a mail from external networks to mydomain.dm reaches my qmail, it not send the mail to the exchange server, but try to send the mail to the MX record (the public address) tha isn't reachable from internal network for nat reasons.     how i can solve this problem? is a nat or qmail problem?