[exim] check_backend router

Hi,

I am trying to set up an exim/MailScanner anti spam gateway which will
receive mail, scan it and send it further.

I am trying to implement a router that when an email is received, exim
will check if the email exists on the remote SMTP server(which gets
the email after it is scanned).

I have the latest version of exim running on CentOS 4.6.

The domains are relayed like this:
domainlist relay_to_domains = lsearch;/etc/exim/relay_domains

Domains that have catch-all emails:
domainlist domains_with_catchall = lsearch;/etc/exim/catchall_domains

Here's what I have so far in the check_backend router:
check_backend:
driver = redirect
domains = ! +domains_with_catchall : +relay_to_domains
allow_fail
allow_defer
forbid_file
forbid_pipe
data =

Then the deliver_clean router follows:
deliver_clean:
driver = manualroute
domains = +relay_to_domains
transport = remote_smtp
route_data = ${lookup{$domain}lsearch{/etc/exim/mail-routes}}


Can someone please tell me what do I put in data = so it checks the
backend SMTP server if the destination email exists? If it does not
then it should reject the email.

Please help

Thanks