I am in the process of migrating to a new AD DS from AD 2000. We have Exchange 2003 in the old and in the new domains. I am wondering if and how it's possible since I will be doing a slow migration (a group of users each day) to allow the users who have been moved to the new Domain and Exchange server to be able to still see and send mail to the other users on the old domain and exchange server. Microsoft KB articles are requested but not required for answers.
Based on your comment to TJ, it sounds like you're looking for a unified Global Address List between the organizations and email delivery (presumably sharing the same Internet domain name) between both organizations.
re: the GAL - Your choice is between third-party software or scripting something up yourself.
If you're going to do "a few users a day", you might be best off exporting the user list from both organizations using something like CSVDE, slicing and dicing that list using Excel, and generating an import for both directories to create mail-enabled contact objects in the opposing directory. In that way, then, you could have a unified GAL.
Each day when you move users, you'll be deleting the mail-enabled contact corresponding to them in the destination organization, creating their new mailbox in the destination organization, deleting their mailbox in the source organization, and creating a mail-enabled contact in the source organization referring to their new mailbox. This could be automated by a script that queried each directory and performed the appropriate creates / deletes of mail-enabled contacts, but you can easily do all that by hand.
That would get you a unified GAL. If you need unified distribution lists between both organizations you'll have to manually keep those in sync, too. A script would handle that better, but you could do it by hand.
Finally, for getting SMTP delivery sharing the same domain name you'll need to configure each of your Exchange 2003 servers to "Forward all messages with unresolved recipients" to the other Exchange 2003 server computer. (You can have the potential for mail loops here, but the loops will be short-circuited by the maximum hop count specified on the SMTP virtual servers. As long as you're filtering incoming email at the border of the network for valid recipients only, this shouldn't be a major issue and something you can live with until you retire the old Exchange 2003 installation.)
If you haven't started yet, I'd seriously consider the need to perform the migration at all. Virtually anything that's "wrong" with your existing Active Directory can be fixed in-place rather than peforming a painful cross-forest migration and email coexistance as I've described above. Getting together with somebody who is very familiar with Active Directory and explaining your reasons for wanting to migrate may shed light on a much cheaper and faster solution to your problems. Leveling the directory and starting over really should be a last-resort measure (getting away from toxic schema modifications, etc).
If you don't mind sharing, I'd be interested to hear what kind of specific problemms you're having with your current Active Directory installation. If it's not something you're comfortable sharing here, I'd be interested in corresponding with you off this site (email, etc) briefly if you're interested. While I certainly take the recommendation of Microsoft re: your Active Directory to have a lot of credibility, it shocks me that they'd recommend leveling the directory unless there are really major problems.
What you're doing is essentially a cross-forest migration of Exchange users. The Exchange Deployment guide has a section of this, find it at http://technet.microsoft.com/en-us/library/bb125074%28EXCHG.65%29.aspx. (Look for "Migrating Accounts and Mailboxes Across Forests")