Sometimes we encounter problems during Exchange transitions due to unpredictable behaviour of email address policies. Usually this manifests itself as users reporting that their primary SMTP address has changed after they were migrated from an Exchange 2003 to Exchange 2007 mailbox server. The problem then becomes reconfiguring their email addresses to be the same as prior to the mailbox move, which is difficult without anything to refer to.
The simple solution for this situation is to export all of the SMTP addresses assigned to mailbox users prior to any mailbox moves. This gives you a simple text list to refer to in any emergencies after the moves have occured.
You can do this with a simple Exchange Management Shell command.
[PS] C:\>get-mailbox | fl name, emailaddresses > d:migrationmailaddresses.txt