Hi Conny,
This error might occur when Backup Exec performs a backup of mailboxes with
having "Single instance backup for message attachment" enabled.
When a mailbox has already been backed up, deleting a message attachment, which is associated with multiple mailboxes will make the job fails with the
error "Warning: Unable to backup the attachment data associated with one or
more messages".
To resolve this issue, uncheck the "Enable Single Instance backup for
message attachments" from Tools > Options > Exchange.
This should resolve your problem.
Please revert if the issue still persists.
Thanks.
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