[Exim] sender_reject_recipients with lsearch

What would be the format to use an lsearch lookup with
sender_reject_recipients (and sender_reject)?

I looked it up in the specification and it said to use an address list,
which I though could be provided
through lsearch.

I've tried, to no avail, the following:

sender_reject = lsearch;/usr/exim/reject.senders
sender_reject = partial-lsearch;/usr/exim/reject.senders

Where reject.senders contains:

certain_spam_users@???:
@spamming_domain.com:

Any help would be greatly appreciated.

Ollie