Hello,
Make sure that the backup exec service account has the following settings:
1 : Member of the Domain Admin group and Local Admin Group if Exchange is a member server
2 : Mailbox enabled accouont, send a test email and open it in OWA to activate the Mailbox.
3 : Make sure that the Mailbox is not hidden from the Global Address List.
4 : Add the account as Exchange Server Administrator in Exchange Management Console.
Hope this helps.....