[Exim] partial-lsearch as filter conditional

ok - here's what I want to do.

All the users will have the ability to put a file in their own home
directory called .spam-reject and that file will contain a list of email
addresses and/or domain names of senders to be rejected. The file might
look like this:

*@spamhost.com
peter@???

Anyhow - I need a conditional - and not sure how to do this where I say

if $h_from: [is in /etc/spammerslist <- this is the conditional I need]
then fail

Using partial-lsearch I guess - how do I write that conditional ?