Backup Exchange 2003 Mailbox Prob.

In order to backup hidden folders in exchange 2003, you have to give some extra privileges at the account you are using to start the remote agent on the machine/s you are running Exchange 2003.

You should try this, open the exchange's system manager and point at your top level icon (organization), then right click on it and select "Delegate Control", press next and ath the "User or Groups" dialog, press add and select the aforementioned account from your domain account, at the role section select "Exchange Full Administrator", press OK and then press next and complete the procedure.
I guess that this will solve your problem.

Try after that to see through your backup program's administrative interface, the mailboxes you have mentioned.

I know that this solution works for the Veritas backup products.

I am using Veritas Backup Exec v9.0

By default, administrators have denied access to user mailboxes, you need to Grant Send As/Receive As rights on user mailboxes to backup them up fully. See server or store properties to change permissions.