Tony,
Your AT&T email account is not affected in any way by this change.
What is changing is that it will no longer be possible to logon to "tumblr.com" with an email address using one of the listed domains.
Since Tumblr is part of Yahoo, my guess is that eventually this will affect other Yahoo features, including Groups. I have not found any other information for Yahoo/AT&T, only the Tumblr notice.
If this does include Groups, then you will need to get a Yahoo email address to access Groups.
I expect that there will be other interruptions as the Yahoo/AOL/Verizon integration continues.
Dave Sand
