Hello. It seems I have lost permission/access to the mailbox store, when I try to make a backup or when trying to access the mailboxs in the store through Backup Exec it gives me access denied, and this is with Local System Account (aka Administrator). I have dubble checked everything, like whoare delegated in the exchange system manager, even directory permissions. The mailbox store database is on another partition e:\program files\exchsvr\mdbdata while the server is running in c:\program files\exchsvr. Can anyone help me here? I cant backup the exchange store right now By the way, the exchange mail systemfor the clients is working good.

Are you seeing anything in the event log with an ESE source? It'd be app log warnings or errors that something else was holding onto the db not allowing the exchange backup agent to access it. You could try a quickie backup with NTBACKUP on the local machine. That should kick you enough errors to see whats going on. If it worked in the past, but not now, you may just need to restart the info store service.

Did you recently install Exchange SP2? Or any other changes that happened right before this issue started?

To do a Brick Level backup of mailboxes the backup software must be using an account that has permission to access users mailboxes. This can be grnated per mailbox put its probably better to grant this permission on the mailbox store object Have this been working before?