Hi,
Please enable the "Restrict Anonymous" key on the cluster nodes as well as on the media server.
How to enable support for "Restrict Anonymous" when using Backup Exec 8.6, 9.x, and 10.0 for Windows Servers
http://support.veritas.com/docs/238618
Also check the rights assigned to the Backup Exec logon account.
It should have the following rights
1)Domain Admin
2)Exchange Administrator
3)Local Administrator for remote server
It must have an unique mailbox to its name and should not be hidden in the "GLOBAL ADDRESS LIST"
Hope it helps.
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