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Also do you have any of this conditions: https://support.ca.com/cadocs/0/CA%20ARCserve%20%20Backup%2015-ENU/Bookshelf_Files/HTML/MS_Exch/index.htm?toc.htm?exchange_unable_to_create_the_user_account_create_the_mailbox_or_finalize_the_mailbox.htm ?
Do you have any error messages in event viewer or when you try to create the mailbox?