[Exim] Processing Spam for Other Email Servers

I have a server running exim that is set up to filter spam and viruses
for other email domains and then pass the mail on to their existing
email server. Right now the way it works is that they point their MX
records to me and I have to add their domain to a file telling the
system where to forward the processed mail once it is done.

Router looks like this:

process_and_forward:
driver = manualroute
domains = +preprocess_domains
condition = ${if match{$h_X-Spam:}{HIGH}{no}{yes}}
transport = remote_smtp
route_list = * ${lookup{$domain}lsearch{/etc/exim/preprocess}{$value}{}}
no_more

And it works great - but I want to further automate it.

What I want to do is if my server is the MX server for the domain, I
want to forward it to smtp.$domain but only if smtp.$domain is defined.
Still trying to figure out all the details of how to set this up but I'm
trying to figure out a way that people can self configure email
forwarding through my system.