Find out from the exchange admin if they have a retention policy in place. That would cause the Exchange server to automatically delete email older than a certain age and you cannot recover it. Also, do you have auto-archive on? Did you turn that on recently perhaps? Check in hidden folders on your pc for pst files, the email may be there. Lastly since new mail sticks around I doubt this is the issue, but do you have your outlook view set to unread messages? That would cause all old read mail to “disappear”. Changing the view to “messages” will make them all magically “re-appear”.