The links now say not listed, however had you clicked the second link (I'm assuming you did not) you would see this:
Results of Lookup
208.74.121.106 is NOT listed
This IP address was detected and listed 17 times in the past 28 days, and 1 times in the past 24 hours. The most recent detection was at Mon Oct 29 05:25:00 2018 UTC +/- 5 minutes
This IP address was self-removed 2 times in the past week.
This IP address was self-removed 1 times in the past 24 hours.
So it looks like there is a problem with that IP address. There is no issue our end; our incoming spam filter has been blocking emails sent from this IP for the reason stated in my original post.