It had nothing to do with migration from Y! to Groups.IO, but rather, something going on from Gmail side. Mark is unbouncing Gmail address users at the moment, which will take hours (paraphrasing his post on Beta group).
Shal, I haven't read other posts, just yours, but I just got the bounce message for one of my groups.? I went to the group to see what bounced, and she reported hundreds of messages bouncing, all gmail.? I sure hope Mark can fix this.? With yahoo closing, I would assume he knew what was about to happen here and had prepared for it.? First time that's ever happened to me in gmail.? Not good. Terri