[exim] Produce 550 error depending on sender's email

Hello All,

What is the best method for producing a 550 error dependent upon the
sender's email address?

We need to send a 550 error to several senders even though the recipient
email address is, in fact, valid. This is because the recipient does
not want to receive any email from the sender and wishes them to think
that their email does not exist on our receiving servers.

An error message similar to this is what we would like to achieve:

"The error that the other server returned was: 550 550 5.1.1
<xxxxx@???>: Recipient address rejected: xxxxx@???."

Any help would be appreciated.

Thanks!