Rick, the users and their mailboxes *were* from a different ORG, but there were migrated - so now they are on this Exchange 2000 server which is part the same Exchange 2003 ORG mentioned. They are not different ORGS.
Nope, you can't use a front-end server from one org to access mailboxes in a different org.
This could change in E2K7 as the 'front-end' servers there are not required to be domain members (thankfully) but I suspect they will need to be associated with the Org in some way.
In any case, sounds like you're looking at additional OWA capabilities.
Now having said that, instead, what if you popped an ISA server into your DMZ? That could be used to reverse-proxy OWA for both orgs (different URLs of course) but I'm not sure if you would get the easy publishing capabilities or have to play a few games to get the second org exposed correctly. In E2K and E2K3 you get OWA automatically on the mailbox server, so pointing ISA at both the original (E2K3) and the second (E2K in this case) server to expose OWA should work.
From: Danny <nocmonkey@xxxxxxxxx>
Date: Tue, 31 Oct 2006 17:51:17 -0500
Subject: [ExchangeList] OWA Access via different Exchange server
Previous environment: x1 Exchange 2000 Server on Windows 2000 Server SP4
Current environment: x1 Exchange Server 2003 SP2 on Windows Server 2003 R2 - OWA, Outlook RPC over HTTPS, mailbox server, etc.
The Exchange 2000 server has been brought out of semi-retirement, shipped to the USA, brand new mailbox stores and storage groups were created, and users from a different Exchange ORG and AD forest have been migrated to this server. Is it possible for remote users on this server to access their mailboxes via OWA? Or is a Front-End Exchange server mandatory?