Router with single IP address, NAT & multiple Exchange Servers

****** BACKGROUND INFO ******

We share offices in a single building.

There is one router connected to the Internet, supplying broadband connectivity to the public LAN (backbone) in the building.

Each business / office in the building has a Windows 2003 SBS Server (all running Exchange 2003) that connects directly to the building public backbone. Let's call them _Server1_, _Server2_ and _Server3_.

Each Windows 2003 SBS Server also has a second network card which is linked to it's own private LAN.

This system allows a level of security and seperation between the businesses.

****** MAIN POINT ******

The Exhange servers all send OUTGOING mail to different external ISP's sucessfully.

The router on the public building backbone uses port redirection to send all incoming traffic on port 25 to _Server1_, where it can be picked up by Exchange on _Server1_. This works fine for the domain controller by _Server1_, eg anyone@abc.com, but means that email traffic for the other two domains (controlled by _Server2_ and _Server3_ ) anyone@xyz.com does not get through.

****** QUESTION ******

How can incoming email traffic be routed to the appropriate SBS Servers / Exchange Servers?

I am farely competent with Windows Server 2003 but kinda new to Exchange so please don't assume I know anything about exchange!

Thanks