yesterday we discovered some very worrying behavior which we would like to share and hopefully we are not correct in our assumptions.
If you are running Exchange on-prem and have users with outlook mobile app (android or ios) with basic auth, then data is cached for 4 weeks somewhere in outlook.com. This came as somewhat of a shock for us, as we are not permitted to store email data in the cloud, however the outlook app does this without the knowledge of the user. The data which is stored cannot be viewed, searched or located. If you are running hybrid auth then this data is cached in your o365 tennant.
Sorry to revive an old thread, but I'm coming across this same situation in my new organization, and I'm hoping you'll see this reply and be able to offer insight on what you did to address this in your environment.