The member on the NL forum is actually a real user from Poland. She receives the error message on her admin email when the user got a 'New private message' e-mail etc, that couldn't be delivered.
But well, if that active user changes her email to a free yahoo/hotmail one, it should be able to deliver the email so the admin won't receive them any longer.
Or turn off her "Receive email on new PM" function in her profile.
* That's what we told the admin to do anyway.