Block before Recipient address rejected: User unknown?

[Juerg Reimann]

Hi everybody,

Is there a way to reject a certain sender email address before he gets a 550 
5.1.1 <em@i.l>: Recipient address rejected: User unknown? When I add the sender 
to header_check, he still gets first the User unknown reject when he sends to 
an unknown user...

Thanks,
Juerg