[Exim] acl_smtp_helo

I would like to reject certain messages based on the "received from"
domain. At first I tried "sender_domain" but that only verifies
(apparently) the "mail from:" address. I am assuming that somewhere
there is a variable that contains the domain that actually "helo"'d.
How can I access this information and ACL it? The Exim documentation
is pretty sparse on this particular ACL form.

Thank you for your time.

--
Aaron Dalton
http://aaron.daltons.ca
PGPKeyID: 0x29844D7D