Exchange NDR Logs

On my exchange server, I keep getting alot of non delivery reports in the logs with strange email addresses, with Event ID either 3030 or 3018.

Example:
A non-delivery report with a status code of 4.0.0 was generated for recipient rc822;u5160286@tknet.tku.edu.tw (Message-ID )
Unless I’m mistaken, this is from spam, but I’ve turned off open relay. What am I missing?