Hello,
Please perform the following steps to troubleshoot this issue:
In order to back up and restore individual Exchange mailboxes, Backup Exec requires access to a uniquely named mailbox within the Exchange organization
-- Create a new User with mail box,
-- add to domain admin groups, give exchange full admin rights, Configure the mail client for the new user
-- activat the account
-- Add the account in Backup Exec logon account
-- Log into Backup Exec using that account and then try running a backup job for Exchange mail boxes
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