[Exim] accepting mail for users based on user AND domain

I host 2 domains on my server (for the sake of argument, call them a.com
and b.com)

Now, if I do

adduser x

at the terminal, the default exim setup will accept mail for x@??? and
x@???. But x exists only at b.com, so I want mail to x@??? to
bounce. How can I do this?

Running exim 3.35 on debian 3 (it's in the stable release and adding from
unstable/testing isn't fun)