Version 9.x we could choose which mailboxes to back up or leave out. 12.5 is the whole store or nothing. I see granular, individual mailbox restoration is possible but not backup?
11D is the same. It's not possible to do backups of only certain mailboxes, and although there is a selection list that offers exclusions, I don't think you can do it through there either.
You can select mailbox individually to backup after enabled "the legacy mailbox support" from option menu. This is not recommended. Backup individual mailbox means backup emails one by one. It takes very long time if you have many mailboxes, the job might takes longer than 24 hrs.