One of the tasks to perform before removing the last Exchange 2003 server in a 2007 migration is upgrading the Default e-mail address Policy.
The method for upgrading the policy is to fire up the Exchange Command Shell and run the following command:
Set-EmailAddressPolicy "Default Policy" -IncludedRecipients AllRecipients
You may recieve the following error: Set-EmailAddressPolicy:The recipient policy "Default Policy" with mailbox manager settings cannot be managed by the current version of Exchange Management Console. Please use a management console with the same version as the object.
This is typically caused by a Mailbox Management Policy being applied to the Default Policy.
To address this: Open Exchange 2003 System Manager, go to Recipients, Recipient Policies and right-click the Default Policy, select Change property pages..., and uncheck Mailbox Manager Settings.
Now run the upgrade script above and get on with removing that last legacy server.