In Exchange 2010 when a source mailbox is moved from Exchange 2003 to Exchange 2010 using the Mailbox Replication Service there is a potential issue can arise that source mailbox is not removed at the end of the move.
All of the content in the mailbox is moved from the source Exchange 2003 mailbox to the Exchange 2010 destination mailbox. All the user mailbox attributes are updated to point to the new Exchange 2010 mailbox
This issue can be caused by problems within the search folders that are located in the source mailbox
This issue will be mainly encountered in Exchange 2003 servers. This issue currently resolved in the Exchange 2007 mailbox servers that have been upgraded to Service Pack 2.
When you try to purge the mailboxes you will get the following error:
"The Operation cannot be performed because this mailbox was already reconnected to an existing user"
The solution is : http://support.microsoft.com/default.aspx?scid=kb;EN-US;930363