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 66.175.222.12 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.

Jim