[Exim] Howto block all unknown/unlisted user?

Hallo,

how do I block mail for unlisted/unknown users on my system?
I think when a email for xyz@??? arrives exim should check wether
the email adress exists. If the adress doesnt exist, exim should block
this mail. I am shure this is possible with exim but I must be a
little bit blind...
Can somebody help me?

Bye
Markus