There was a post yesterday regarding this discovery on GMF.
The only thing that can be done is to send emails to:
abuse_rbl AT abuse-att.net <<< (the email address cannot be hot-linked here 🙄)
In your email, you should request that they please whitelist the Groups.io email server, web01.groups.io at 220.127.116.11 because it is a legitimate server used by tens of thousands of users and groups.
The more people who send an email, the sooner it will be resolved, IMHO.