[Exim] routing only non-local mail to spamassassin

I currently have the following router statement (using exim 4.22):

spamcheck_router:
  driver = accept
  transport = spamcheck
  local_parts = lsearch;/etc/mail/spamusers
  no_verify
  condition = "${if and { {!def:h_X-Spam-Flag:} \
                          {!eq {$received_protocol}{spam-scanned}} \
                          {!match {$sender_host_address}{/^129\.246\./}} } {1}{0}}"

I'm trying to only check for spam if the email is not coming from our network (129.246.0.0/16). I think $sender_host_address is always blank for some reason. Anyone have any ideas how I can do this?