If you use other e-mail software or POP3 retrieving function in other free e-mail services (sometimes you are not aware of it), e-mails in the inbox may disappear. An easy way to investigate the problem is to change the password of your ECS-ID and see that the problem occurs again. If the problem continues, please consult the support desk of IIMC. You may be able to recover e-mails deleted from "Recover deleted items..." shown by right-click the "deleted items" in the inbox of OWA.