[Exim] host_address and smtp

I have a need to strip or not strip certain headers, depending on the IP
address of the host which I'm transferring the message to via an SMTP
transport. headers_remove is expanded, which is great, but unfortunately
$host_address is not defined for remote transports. I can't think of any
particular reason why that should be so, but so it is. Anyone have
thoughts on a clean and efficient means to accomplish what I've described?

--
Pete Naylor