Unable to list the Exchange mailbox users

While configuring mailbox backup to cloud backup server from my client instance the list mailbox backup configuring page is showing the error message as "Unable to list the Exchange mailbox users.Make sure sufficient permissions are available for the user account under which VembuOnline backup service is running.". I am not sure what kind of privileges to be given to the user and is that any other client level changes needed to be done in order to fix this issue. Any idea on how to fix this issue.

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