They block and don't inform
They give a URL for providers to get unblocked. I used it, they said they unblocked us, and then blocked us again. I used the form again and their robot said "you're not blocked." And a blocked user got their bounce header to us showing we were in fact still blocked. AT&T provides no means to determine WHY an address is being blocked, and what with their refusal to unblock us because they don't know they're blocking us, I think it's time to float a class-action lawsuit.