Extending my last suggestion, because I more carefully re-read the requirement:
> 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.
How about this ?
acl_check_rcpt:
deny message = [2C03] Unknown email address $local_part@$domain.
recipients = jonny@???
senders = EXDIR/jonny.unwanted
EXDIR is a predefined variable pointing to the location of Exim data files.
jonny.unwanted contains
sally@???
fred@junk,com
--
Paul.
England,
EU.