This is the second notice I have gotten. It is from... c/o The Garden City Group, Inc. ("GCG").
Nwabueze v. AT&T, Case No. CV-09-1529 SI
Seattle WA

It has a phone number of 1-866-242-0603
info@ATTthirdpartybillingsettlement.com
It says that it is a court-ordered notice.. read carefully... leagal rights may be affected... and not a solicitation from a lawyer.
It says I received this notice because AT&T's records indicate that I may have been billed for third-party charges on an AT&T landline Telephone Account between 2005 and 2013, and if we did not authorize those charges we may be entitled to a payment from this class action settlement.

Have you heard of this?
I'm always very skeptical of this stuff, and most of the time I just glance at "junk mail" for half a second and chunk it in trash right away, but this looks like it may very well be legit.