"All mailboxes" should be an opt-in feature!
If you do work and non work on one device then you don't want commingled views and any risk of sending mail from the wrong account or to the wrong people!
Kenneth W Gourlay commented
At the moment, when an email is received from one of multiple mailboxes, Outlook switches to view that mailbox only. This behavior should be changed such that to the all mailbox view is maintained unless specifically selected by a user.
Robert Niedermair commented
Add a visual element to mail objects in "All Mailboxes"-View, so that the user can distinguish in which mailbox the element is stored. Only relevant when using multiple mailboxes.
Samsung Mail offers a similar function which is very useful