This reminds me of the migration from old to new when Adelphia switched to Comcast. Some were still on the old Adelphia settings while others were on the new Comcast e-mail settings.
While AT&T to my knowledge has not gone through a major restructuring, they may be performing a server hardware migration from old to new and to make the old hardware work along side the new its best to have different settings and then move controlled groups of customers over to the new servers at a time until all are eventually migrated over.