I have a user whose email before yesterday has vanished from their inbox. Looking at ALL mail and sent mail, which shows some items labeled INBOX that don't show in the Inbox. Other than marking said messages - unread (which is not the case) I'm not sure why they don't show in the Inbox.
Checking my own account - messages I reply to show up both in my sent mail and in my inbox.
Messages this user replies to only show up in the sent mail container.
However, this user also checks their email by phone - so I wonder if that is where the problem lies? I would appreciate other's thoughts on this.