This is currently using one Exchange 2003 server, to host 2 external email domains - a.com and b.com. Coincidently, my internal 2003 AD Domain is named a.com (FQDN). There are about 120 email users, all of them using a.com email domain; only 12 users have both email domains (a.com and b.com).
Recently, there is a change on company policy - these 12 users that using b.com, have to be separated. The direction is, a new Internet access network that comprises of separate Internet line, firewall, and related network equipments, are formed. Sooner or later, these 12 users have to be "logical" move into this newly-formed network, probably using new domain user account, ip addresing, etc.
Let's called this newly-formed domain b.com (Internal AD Domain). The interim solution is these 12 b.com users still using the same Exchange server on a.com. My challenge is how can assure that all these users can continue to use their b.com email domain mailbox with little interruptiion to their normal operations?