*** Updated on 11/21/2007 by Wrobinson ***
Make sure that the LegacyExchangeDN is being migrated for the users. This is how Outlook addresses messages to users in Exchange. Please refer to the following article http://www.msexchange.org/tutorials/Understanding-LegacyExchangeDN.html.
*** End Update ***
do you still have the exchange 5.5 online? If so ask the users to retrieve their mail to local pst files. Delete the mailboxes and recreate them again. There seems to be a specific problem in the migration process!!
What is the size of those mailboxes? Are they larger than the other successfully migrated boxes?