System emails go to the address that is configured under primary settings in the control panel. Do you check and test this address ?
I notice on my board some tries to register 'spam accounts' to harvest email address or post comercials. These accounts are most time created with one-use emails services and bounces comes back to your board email.
I send out every month a newsletter and get some back. this is the point to check out these accounts and delete or notice them with pm's.
