Are you saying Time machine will not backup mail ? ( user/library/mail )
No, but recovery is much more difficult and has been for me and others for awhile.
You used to be able to go into Apple Mail itself, click on a mailbox and then use TM to go back and recover. That doesn't work all the time, often it doesn't work at all. It used to.
Now if you want to go back, you have to futz with the individual mail items, if you can find them, outside of Apple Mail.
This has been discussed on the Apple support forums for ages, it's still broken, at least for me and others. Perhaps not everyone and again, this goes back to the Adobe bug that's difficult to find and fix because it's not affecting the majority of users.