How to configure ISA 2006 with Exchange 2007

Dear All,
I have and Exchange 2007 configured and working properly.
I installed ISA 2006 and published the exchange server to use an public IP www.xxx.yyy.123 the ISA server have two network adapter one internal and one external configured with www.xxx.yyy.122 when exchange relay messages it appear like it sent from www.xxx.yyy.122 instead of www.xxx.yyy.123 is there any way to configure ISA 2006 to use the correct IP address www.xxx.yyy.123?
Thanks in advance for help
Amgad