EXMerge Error : MAPI_W_PARTIAL_COMPLETION

When you use the Microsoft Exchange Mailbox Merge program (Exmerge) to export mailbox data, you may notice that some messages are not exported to the personal folder files (.pst files). As a result, the .pst files are smaller than the associated mail boxes in the Priv.edb database, and you may find a number of the following entries in the Exmerge.log file generated by the Exmerge process:
Error copying message with subject ?SUBJECT?
(MAPI_W_PARTIAL_COMPLETION)