[exim] Block outgoing to specific domain

Hi,

I was wondering if there's a simple way to block outgoing mail to a specific
domain such as aol.com hotmail.com etc. I'd like to do this for all users,
in the case that there's a complaint that we need to investigate and hold
all email to that domain for the interim.

I've Googled and seem similar questions but no real answers that I could
follow in the right direction. Can anyone kindly give me a hint :-)

thanks....