Exchange 6.0 Primary Email Addresses

Have a <20 user LAN with W2K Server -DHCP- and Exchange 6.0. We have control of two different domains, e.g. xy.com, and zz.com, and will be using them for both email -on ms exchange- and webhosting. Some of our users need xy.com as their primary email address-the address that appears in the 'from' field- , some need zz.com. The problem is, users are only able to send messages if xy.com is tagged as their primary email address. I'm guessing this is a problem within ms exchange but I've looked everywhere and can't find it. One more note: Everything works fine within the lan. Thanks in advance.

Win2K Server sp2
MS Exchange 6.0
Netopia 5100
Ded. T1

Hope this is enough info.

Never Mind. Figured it out.