Johan Veldhuis blogged about the problem we encountered when moving mailboxes cross-forest from Exchange 2003 to Exchange 2010 (also see my 2-part article on cross-forest move). We received the following mailbox move failure message:

Warning: Unable to update AD information for the source mailbox at the end of the move.  Error details: An error occurred while updating a user object after the move operation. –> Failed to find the address type object in Active Directory for address type “SMTP:AMD64″.

Failed to cleanup the source mailbox after the move.
Error details: MapiExceptionNotFound: Unable to delete mailbox. (hr=0×8004010f, ec=-2147221233)

So, besides an Exchange 2010 mailbox the Exchange 2003 mailbox was still there, and AD attributes weren’t changed on the source AD object (e.g. HomeMDB still pointing to Exchange 2003). This is a potential serious condition as incoming e-mail might be delivered to the Exchange 2003 mailbox instead of the new Exchange 2010 mailbox, depending on your setup and speed to remove the source mailbox.

To assist with this issue Microsoft released hotfix 940012 for Exchange 2003. It will generate an event on Exchange 2003 when it cannot remove the mailbox. It will also log which folder might be the culprit. After detecting and identifying  the mailbox with issues, you still need to remove it manually (steps should be known; if not, they’re contained in the kb article).

You can view kb article 940012 here and download the related hotfix for Exchange Server 2003 SP2(!) through here.