Okay, so we id-d that it seems to be IMAP related. Now, we need to all submit independent support requests to flood MS with the problem in Hope's they will fix it.
To do so, in Android at least, in the Outlook App, click on the top left icon (may be a photo you selected for your profile), then the help icon (a ? in a circle) at the bottom left, then "Contact Support". It will come up with prompts on "How can we help?". I suggest "performance/crash" the "other", then complete a submission.
If we all do this, perhaps it will be resolved. We must flood them with it.
For your consideration, I gave this as my submission. Feel free to just copy and paste.
Cheers!
My submission:
MAJOR BUG REPORT: The app has a serious flaw. When commerce to an IMAP account, at least on Android, the app downloads a significant amount of the email in that IMAP account, resulting in the app using 1GB or more of data storage on the phone. The only solution is to delete the IMAP account. Resetting the IMAP details yields again the same problem within a day or two. The long term workaround it to avoid using IMAP in this app, pending this bug is resolved by Microsoft. Please pass this bug report to the developers. More details at
https://answers.microsoft.com/en-us/outlook\_com/forum/osettings-oemailset/outlook-android-storage-use/a9733ad9-c44a-4c28-b4f2-fb6f73f34fcd?messageId=e2f60912-ca6d-4db9-bf0c-a78ebc642c9c