Restoring mail folder from Time Machine backup

I have backed up my Mac to Time Machine and reformatted it. I can see my old backups on my backup disk. When I open Mail and then Time Machine to try to go backwards to view my old messages I am unable to go back beyond the previous day when I setup a new Time Machine backup on the reformatted Mac. How do I access older mails please?

Another option is to open the backup disk and open the Mail folder within Library. But when I try to do so the Library folder, though unhidden, is grayed out and won't open! Any help appreciated