Hi,
A uniquely named mailbox for the Backup Exec service account must exist on the Exchange server. A unique name is a name that is not already in use by the organization, that also is not contained as a portion of another mailbox name (i.e. "mailbox" would not suffice as a unique name if there were also names like "mailbox01," "mailbox02," etc. on the same system.) The mailbox must not be hidden.
In order to isolate the issue, please refer to the following technotes:
- Preliminary Troubleshooting List for Exchange and mailbox backups
http://seer.support.veritas.com/docs/242606.htm
- How to protect Exchange 2000 and 2003 servers using the Backup Exec for Windows Servers Exchange Agent
http://seer.support.veritas.com/docs/266074.htm
- How to configure the Backup Exec service account to have sufficient rights to connect to an Exchange 2000 server
http://seer.support.veritas.com/docs/234575.htm
We hope this will help.