I have used the whole time the Outlook App for iPhone (iOS 12), and it seems when installed - my Battery doesn't last that long with it. Only 2 Accounts are synced with it, also with OneDrive signed in it seems that battery suffers - AND I do not have any background refresh enabled.
Can it be the Spotlight indexing ? (because my OneDrive and Outlook contains a lot of files, that are not all indexed?) - my phone is 12 days on iOS 12 (new iPhone XS Max).
Can it be the Spotlight indexing ? (because my OneDrive and Outlook contains a lot of files, that are not all indexed?) - my phone is 12 days on iOS 12 (new iPhone XS Max).