As taken directly from the Backup Exec Admin guide (Agent for Exchange server section) available on the product CD:-
Using Backup Exec Logon Accounts with Exchange
In order to back up and restore Exchange Server resources, you must use a Backup Exec service account that has domain and local administrator rights on every Exchange server that you want to back up.
In order to back up and restore individual Exchange mailboxes and public folders when
using Exchange 2000 or 2003, use Microsoft?s System Manager utility to grant the Exchange Administrator role to the user account at the Administrative Group level (for all Administrative Groups if there is more than one). In order to back up and restore individual Exchange mailboxes and public folders when using Exchange 5.5, use
Microsoft?s Exchange Administrator utility to grant the Service Account Admin role to the user account at the Site level (for all Sites if there is more than one).
Additionally, Backup Exec must have access to a uniquely named mailbox within the Exchange organization. When selecting mailboxes or public folders for backup, Backup
Exec will attempt to find a mailbox with the same name as the username stored in the Backup Exec logon account used to connect to the Exchange server. If you use a Backup
Exec logon account that stores the credentials of a user account that is unique and has a corresponding mailbox of the same name, then you are not prompted for an additional
logon account when selecting mailboxes or public folders. Otherwise, you must choose or create a Backup Exec logon account that stores the name of a unique mailbox within the
Exchange organization. A unique name is one that does not share the first five characters in another mailbox name. For example, if EXCH1 has been entered as the mailbox name, and there are other mailbox names such as EXCH1BACKUP, or BACKUPEXCH1, then Backup Exec will not
accept the name and you are prompted to choose another mailbox name. If you cannot create a unique mailbox name, you must create a Backup Exec logon account and enter the
fully qualified name in the username field of the logon account.
For example: /O=Company/OU=Orlando/CN=Test/CN=EXCH1