E-mails in inbox Disappear

I have already 3 users that has complaied that when they click on a new e-mail in inbox, it just disappear form the inbox, but it is still in the All documents folder. Why is this happening, I did checked all their settings, and it looks right. How can I fixed it

Please ingnore thismessage, I have found the problem.

Subject: e-mails in inbox Disappear

Whenever I’ve had this issue the user has the button clicked to just “show unread”.