I have been getting the messages from 1111302000 also and called ATT tech support (1-888-892-9760).  After several calls and much "research" on their part they were not able to resolve the problem.  As others have mentioned, they first said it was spam and that I should reply with "STOP" in a message, don't bother, it is not spam, as others have pointed out it is coming from XpressMail's push server.  I was able to ssolve the problem by disabling XpressMail's push feature and have not been getting any more of these annoying messages.  Open XpressMail Settings, Update Preferences, uncheck "Enable Push" and then click menu and then "Save".

Glad I'm not getting the messages but I want to get emails pushed!  Please call ATT tech support at number above, tell them your problem, the temporary solution and ASK THEM TO FIX THE UNDERLYING PROBLEM!  Perhaps if enough people call they will come up with a real solution.