A few years ago one of the reasons people were reluctant to reboot their computers was because they did not want to lose their current collection of browser tabs.
Since then, however, browsers managed to learn to remember your browser session and to offer to re-open your last collection when you restart.
Some Outlook users use open Outlook Message windows in the same way, to remember which emails to respond to, and this caused a significant problem if Outlook or your PC crashed or needed to be restarted.
Microsoft appears to have recognized the issue, as a new feature in a beta version of Outlook is to quickly reopen items from the previous session.
We added an option to quickly reopen items from a previous Outlook session. Whether Outlook crashes or you close it, you’ll now be able to quickly relaunch items when you reopen the app.
This feature will be on by default, and if it really annoyed you, you can turn it off by going to Options > General > Start up Options.
The feature is currently rolling out to the Office Insider beta channel, and should come to regular users in due course.