This error may be reported in the log file during migration, the message will most likely still have been migrated regardless.
This error indicates that one of the email addresses on the mail message were not able to be located in the Exchange Global Address List, the message should still have migrated however.
Look in the Exchange mailbox to confirm that it did get transferred successfully. Verify that the address listed in the error is stamped on a User object in Active Directory, if not add it to the appropriate account and re-migrate the data to resolve the issue.
Please see the NME_PARAMS_REF.PDF section on "RemigrateMode=" for more information on re-migrating.