[exim] Feature or Solution needed - forwarding verify reques…

ok - I do front end spam and virus filtering with exim. I have no access
to most of the servers I process for. But - I want to be able to verify
recipients for these remote users. Here's my router.

# This router cleans spam and forwards it to another email server.

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

If another server tries to verify for any of these domains it returns
true for any email address.

What I need is for my server to "pass on" the verify to the target
server (returning verify if the server deferes or times out) so that I
can let the sender know if the account exists remotely.

Or - if someone has a better solution?