[Exim] sender_verify_callback & logging: 3.36

I have

------8<----------
# Attempt to verify incoming mail from commonly forged domains.
sender_verify_hosts_callback = *
sender_verify_callback_domains = lsearch;/usr/local/exim/callback_domains
log_refused_recipients = true.
------8<----------

in my Exim 3.36 config file, Solaris 2.5.1.

I am not seeing anything logged about callbacks, either succeeding,
or failing.

I expected to see some evidence of the callback taking place, at
least in the failure cases. Are my expectations wrong?
I can see nothing very relevant among the /log_.*/ options.
I suspect I've missed something important.

        Thank you
        Hugh