Here are some screenshots of the debacle [edit: properly redacted now]
It started out when someone wrote to that mailing list with
> This is a test to see if external users can email AT&T's internal email list with the most recent email mishap.
followed by other responses like
> I guess so…
> So far, based on the number of unique domains coming back directly to me (and replying all), it appears to be mostly business customers. I am assuming business because I also used my gmail for my business when I set up an AT&T account. Time will tell of course but there's definitely not an insignificant number of people on this list as far as I can tell.
> We’re just lucky there aren’t multiple people with vacation responses on eh?