Update 09:40 MT Nov 18: This issue was resolved as of 16:00 MT yesterday, Nov 17. You may still be seeing these events posted on Event API, and can ignore these as well. - Jacob

If you get an alert regarding a Bounce to this address, or you see this on your Email Reports > Bounces, you can ignore this. This is a minor bug with one of our internal servers, and we're working to resolve this right now. - Jacob