Richard,
I am curious as to why it is showing that the failure message is
coming from Yahoo.
Because under the hood Yahoo Mail is providing email service to att.net (and SBCGlobal) customers.
Or were. I thought in reaction to the purchase of Yahoo by Verizon that AT&T had decided to pull its users off of Yahoo Mail and substitute their own service. Or maybe not yet. Or maybe I misunderstood.
Is ATT/SBC blocking io Groups?
Probably not. There are a half-dozen GMF members with att.net email addresses who are not bouncing.
In this case it is as Duane mentioned. When the member tried to post to Groups.io, his message outbound from att.net, by way of Yahoo Mail, was rejected by Groups.io. Yahoo Mail reported that fact back to the member.
How that member's email address came to be bouncing is something you could investigate through his row in your group's Members table. Click anywhere on the row except the checkbox, and on the page that opens look at his Email Delivery History tab. That should say something relevant to why his address has bouncing (blue B) or bounced (red B) status.
Shal
Shal Farley