whitelist_from_rcvd when rDNS fails

Hi,

I need to whitelist a sender, and I typically use whitelist_from_rcvd,
but it's not working in this case, and I suspect because rDNS fails:

   Received: from ideascollide1.ablehost.com (unknown [208.81.177.83])

Is the next best approach to create a rule that deducts points or is
there another way? There aren't any DKIM headers to work with either.

Thanks,
Alex