Deleting downloaded messages
Apps and devices that use POP can be set to delete messages from Outlook.com after download, which means you won't be able to read your email in Outlook.com.
Don't let devices or apps delete messages from Outlook.com. If the app or device is set to delete, Outlook.com will move the messages to a special POP folder instead.
Do what the device or app says—if it's set to delete messages, delete them.