I have a client and one of the users deleted mailbox is constantly being flooded with blank email (no sender, no body, no header) everyday. I have checked mailflow and they do not show up in the logs. I'm curious if anyone else has experienced this issue? I am open to any suggestions as because this has been going on for months now. The user is using Outlook 2016 but the deleted items also show up in OWA.
If there are no traces of these messages in the message trace logs, it means they are being generated by the client used, or perhaps some mobile app using the mailbox. One way to get more information would be to enable auditing (owner events included) and go over the audit logs. Or simply remove the mailbox from any apps/devices and test it by only using OWA for a day or two.