Postfix domain re direct

[Madmax2]

Madmax2 here
I have a postfix server Ver 2.1.5 working like a dream
I installed exchange 6.5 on Server 2003 R2 in active directory
exchange and all works
when I send mail from exchange and use postfix torelayit the mail address that gats delivered is the local domain and not the host domain of the postfix box.

ea send mail from user@north.nashua to postfix nashuanorth.co.za fqdn on internet and receive mail as user @north.nashua.co.za

I have checked the main.cf and myorigen = nashuanorth.co.za
please help

[Madmax2]

Got it
I used the Address rewriting and Masquerading

Changed the address masquerading to $mydomain
And it seems to work