What follows is not aimed at Don, specifically, but needs to be restated every once in a while.
When Groups.io sends a message to you saying your messages are being bounced, it should contain what is often called an "unbounce link" that you are asked to activate to fix the situation.
That is all you need to do. Test messages are not necessary.
I could have sworn that Groups.io also sent you a confirmation that you're no longer bouncing after activating the unbounce link, but I could be wrong about that.
Brian - Windows 10 Pro, 64-Bit, Version 20H2, Build 19042
Always remember others may hate you but those who hate you don't win unless you hate them. And then you destroy yourself.
~ Richard M. Nixon