Earthink is definitely blacklisting all of groups.io.  
I have about 15 earthink.net members.  The latest soft bounce was 12/16/2018 
The last hard bounce was 12/15/2016
Our message board is fairly active with at least a couple of digests per day.
Something else is causing it the issue to be specific to some groups?
--

Bob Bellizzi

Founder, Fuchs Friends ®
Founder & Executive Director, The Corneal Dystrophy Foundation