If your problem is related to the fact that the Mail App is only downloading/syncing x number of days/weeks/months worth of emails - then you can set the "Download emails from" option to "Any time", which should download and then keep all your emails on the device.There has to be a way of setting the Microsoft Mail App so it NEVER deletes its inbox??
As joeka said, you can adjust the window of time that the Mail app is set to display. The important thing to remember is that the Mail app is not deleting your messages, it's just showing you the messages from the time frame you set. All of your messages are still available either by changing the display period within Mail or using another client (Outlook, Web mail, etc.).There has to be a way of setting the Microsoft Mail App so it NEVER deletes its inbox??