Your email service provider is what bounces Groups.io e-mail messages, and if using the "unbounce request" doesn't resolve it, then you need to contact your email service provider and ask them to whitelist any domain that ends in groups.io.
I recently posted about issues with Earthlink blocking Groups.io messages as spam.
If customers complain, and make it clear that Groups.io is a subscription e-mail list service, these issues with blocking/bouncing are likely to get resolved both more rapidly and more permanently.
Brian - Windows 10 Home, 64-Bit, Version 1809, Build 17763
A great deal of intelligence can be invested in ignorance when the need for illusion is deep.
~ Saul Bellow, To Jerusalem and Back