[Exim] Using sender_host_reject

Hello everyone,
I am trying to add a line to my exim.conf file but it does not work and I
Exim will not accept it.

sender_host_reject = lsearch;/usr/exim/tables/banned_sites

Perhaps I am trying to add it to the wrong part of the exim.conf file?
Where would this statement go? At the beginning or as being part of an ACL?

Thanks for all the help!