How can I move our user accounts from one domain to another without losing the users' mailbox data?

To start the mailbox migration, begin by backing up each user's mailbox data to a .pst file (if you're migrating several mailboxes, consider using the ExMerge utility instead). After you create the backup, use the Microsoft Management Console (MMC) Active Directory Users and Computers snap-in to move the user's mailbox to the destination Exchange server (this step doesn't move the user account, so the Exchange server can be in either the source or target domains). After you migrate the mailbox, use the MoveTree utility from the Microsoft Windows 2000 Server Resource Kit to move the user account to the target domain. The Microsoft article "How to Use the MoveTree Utility to Move Objects Between Domains in a Single Forest" (http://support.microsoft.com/?kbid=238394) describes the MoveTree utility and how to use it. After you use MoveTree, the user should be able to use the migrated account to log on to the mailbox. Of course, you might already have migration tools (such as those that Aelita Software, BindView, and others offer) available. These tools generally do a good job of automating mailbox moves.