[exim] Sending 4xx errors for certain domains

Hi list,

I run a few largeish backup mx servers for a lots of customer domains,
sometimes the the customers do something stupid like limit the number of
mails my mx servers can send them in an hour, or outright block my mx
servers from sending to them, in these situations I'd ideally like to be
able to send 4xx errors after the "RCPT TO" until I can resolve the
issue with my customers, can anyone suggest a good way to do this?

I'm running exim 4.68 on these mx servers at the moment.

Thanks,

--
Peter Farmer