Outlook lost emails

I set up an account for my son when my wife was pregnant so I could email him through the pregnancy and the birth and now he is 6 months old. I've sent him another email today which had a failed delivery notification and then went to check it had been received in the account and noticed that there was only the outlook.com welcome message. After this I resent the email and it then appeared in there but all the other emails have disappeared. I have looked on these forums and none of the 'reason for messages disappearing' are valid - ie. not being active for over 270 days, manually deleting them, being 'grouped' etc etc - none of these are valid so where have the emails gone...!