Access Denied Errors when backing up Exchange 2007 on BE11d

Hello, Scenario as follows:

Backup Server: Windows 2003, Backup Exec 11d SP4

Remote Backup client with problem: Windows 2003 x64 SP2, Exchange 2007

Errors: V-79-57344-33928 - Access Denied. Cannot backup directory MBX1 and its subdirectories.

This error appears for every other mailbox database and storage group that gets backed up on this server.

Backup Account used is a member of Domain Admins.

Turning off the GRT feature on the backup makes no difference.

Seems to be lots of info on access denied errors but jsut trying to make sense of it all and any help would be appreicated.

Thanks