[Exim] Smart Host on a different port?

Exim 4.20

I've got the following router...

send_to_smart_host:
driver = manualroute
route_list = !+local_domains smarthost.domain.name
port = 2025
transport = remote_smtp

However, exim says the port is illegal. How do I get it to smart host the
information to the smarthost using a port other than port 25.