[exim] Help with log message: failed to find host name

Hi all. I happened to notice that I'm adding a lot of 'failed to find host
name' messages in my log, such as:

2015-12-17 13:22:10 [872412] list matching forced to fail: failed to find
host name for 68.171.219.203
2015-12-17 13:22:14 [872465] list matching forced to fail: failed to find
host name for 170.130.206.13
2015-12-17 13:22:15 [872485] list matching forced to fail: failed to find
host name for 14.175.26.108

What's odd is that quite a few of the IPs listed in the log really do have
proper rDNS (including all of them in the example above). When I do an
nslookup on the same server, they all resolve fine.

Any pointers as to what might be going on here?

- Scott