[exim] Smart Host routing except for certain messages

I have a cPanel/WHM host and it has been set up to route all email except
for local messages via a smart host. I would like to modify the routing so
that certain messages identified either by a particular message header or
the "to" email address are not routed to the smart host, but are served
locally or by another smart host.

I have been looking at the documentation for months, but I'm really not
sure how to proceed.

Here is the section of the config file that does the routing:
send_via_sendgrid:
driver = manualroute
domains = ! +local_domains
transport = sendgrid_smtp
route_list = "* smtp.sendgrid.net::587 byname"
host_find_failed = defer
no_more

exim 4.85

Can anyone give me some examples (with explanations) of how this can be
done.

Thanks in advance,

Ken