[exim] Exim used as a filter - want some hosts to be deliver…

We use our exim install as a spam filter, it does a great job, however
some spammers have learned they can send their junk to mail.gmail.com
and it gets past the filter.

In exim.conf, we have following lines in the router section:
internal:
driver = manualroute
domains = +relay_to_domains
transport = remote_smtp
route_data = ${lookup{$domain}partial-lsearch{/etc/exim/transport}}

then we have a file /etc/exim/transport containing:
gmail.com: mail.gmail.com
google.com: mail.google.com

I want mail being delivered to gmail.com to go to port 2525, mail
going to google.com goes to port 25

Please help &/or point me to documentation that would accomplish this.

Thanks,

John