How do I set up email storage in Office365 mailboxes?

Question

Microsoft Exchange

Hello.
There were a couple of situations that led to a stupor, tell me where to find a solution _))
1. The user deleted letters from 2016, you need to restore them.
A policy is used that the final deletion of messages can only be done by the administrator, i.e. where they are stored.
The question is where and how to restore them?
ps the user is active (with a license).
2. Needed letters from a user who was fired a long time ago (since 2016, it has been in a state without a license and with a blocked login). I connected a license to him and gave him the right to enter - he logged into the system, and the box is empty.
How to set up permanent storage of letters in the mailbox of laid-off employees?
p.s. Enable Litigation Hold?
I would be very grateful for help