Hi Franc,
First, welcome to the forums!
- When the mailbox is backed up, does every subsequent incremental backup becomes a sort of restore point? I know it's not the same thing as B&R, but can I go back to the state of the mailbox from 3 days ago, for instance?
You can go to a specific "restore point" of 3 days ago and see what is in the mailbox (just select a specific date) but in reality, as you said, it is not like B&R, it is more of an archive with an applied retention on it. Note that the retention is taken from the exchange online item. So for example, imagine the retention is 3 years, but in your mailbox there is still a mail that is 5 years old, it won't be protected by VBO as it is older than the retention range.
- What's the logic behind a deleted mailbox. When I delete it from O365 and it stops being backed up, will I still have access to the old mail for the duration of the retention period? Or should I extract it and save it somewhere else?
You still will have access and it will remain there until the retention period is over.
Hope it helps
Mike