Upon opening a new session of Outlook, ask user if they would like to restore windows from last session.
Reopen windows from last session
Recovery Button for Open Emails
It used to have this functionality with desktop versions (i.e. 2016 and prior by adding the /restore flag to the command line shortcut) but it has been removed from 365... because I guess it was too useful?
Aaron J commented
The fact that Outlook *cannot* remember windows between sessions and restarts actually causes me to postpone patching and restarting until it's convenient for me to tend to however many dozen windows I have open. In my case, that can be days -- and I doubt I'm alone. If Outlook could remember open messages/etc, Windows patching adherence and overall security would probably benefit.