For the Archiving Option, the Backup Exec service account must access mailboxes on the Exchange Servers that you want to archive. To gain this access, the Backup Exec service account must have permission to access the Exchange Servers.
You can use either of the following methods to grant the permissions that the Backup Exec service account needs to access the mailboxes on Exchange Servers:
The Backup Exec service account must also have the Send As permission on the mailbox that you create for Backup Exec's exclusive use. This mailbox, called the system mailbox, must be created on each Exchange Server on which you want to select mailboxes for archiving.