[exim] Smart Host routing except for certain messages

I submitted this message earlier (2 hours ago) from a different email
address, but I haven't seen it come through the list yet. Sorry if it
comes through twice.

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