Missing Emails - Exchange 2013 - Outlook 2016 - iOS

Hello,

I have a issue with missing emails. A user called me today saying that an email had popped up on his iPhone then in Outlook 2016 then immediately disappeared from both devices. I checked Deleted Items, Junk and even the Sync Issues folder and the email was nowhere to be found.

I logged into OWA and the email was also missing. Another email came in while I was logged into OWA and that too immediately disappeared. this does not happen to all emails and he still has a lot of email traffic with a 17GB Mailbox.

It is a small network with 25 users 1 DC and 1 Exchange server

I have repaired all the OST's associated with his account but he is now concerned he is missing a lot of emails and as always he's the MD.

I would like advice in how to proceed, all I can now think of is that there is a problem with the database but it seems a bit drastic to move all the users to a new database.

Any advice is welcome.

Thanks

Dave