-- Mail Routing --
The routing of mail to Exchange recipients is done with the LegacyExchangeDN attribute. Messages sent are stamped with the senders LegacyExchangeDN.
-- Migrations --
With any version of Exchange, when the mailbox is moved, it's LegacyExchangeDN is overwritten to represent the new location in the directory. Because of routing explanation above, this breaks replyability. Normally, this is fixed by adding an X.500 proxy address that represents the LegacyExchangeDN. Exchange 2003 SP1 does this to all mailboxes moved between administrative groups.
-- Disaster Recovery --
A great article on the role that the attribute LegacyExchangeDN plays in a disaster recovery scenario.
MSExchange.org
