ghostly Outlook Inbox

corporate environement, one and only one user experiences the following :
he claims he does not receive messages, and the fact is that the messages are not showing up in the Inbox, but if I do a search, they come up inthere all the same. Also the number next to “Inbox” shows 300 or so messages, although there are only 15 or so visible in there. Size also doesn’t match, shows way more than the sum of the visible messages. No rules, no autoarchive …