[Exim] Logging sender hosts without names?

I'm now trying to log any sender hosts without DNS names (PTR records).
I've wrote like this:

host_reject_recipients = "+warn_unknown:!*"

Is it the right way? I could not find an answer in the Exim FAQ, so I'm
asking....

Thanks in advance.

--
"Free software is not for free."
Kaz Sasayama <Kaz.Sasayama@???>; Screen Name: kazssym
Hyper Linux Systems (Hypercore Software Design, Ltd.)
<URL:http://www.hypercore.co.jp/>