[Exim] Q: temporarily prevent emails to a particular domain?

Hi,

How would you solve the following problem? There is a mail server
"mail" (runnning exim3) and a client "client". I will be replacing
client, but I'm worried that when the mail server will send email the
new client when it is still in an unstable state, the client will not
accept email.

Given that I have root access to both all machines above, how can I
temporarily prevent emails going from the server to the client. For
instance, can I automatically freeze emails to a certain domain name?

What is the right way to do this?

regards,
Richard