Sprint was the first carrier in the United States to release Marshmallow for the Galaxy S6 and Verizon soon followed suit. T-Mobile has already released said update as well so it's only AT&T that's left and Samsung has confirmed that Marshmallow will soon be out for the Galaxy S6 on the second largest mobile carrier in the United States.
The support page has been updated to show that software version G920AUCU3CPC2 is rolling out for the AT&T Galaxy S6. AT&T is yet to update its page for the handset or issue a confirmation but Samsung's support site has made it clear that the update is out now for the Galaxy S6 on AT&T. No word as yet on when Marshmallow will arrive on the Galaxy S6 edge on AT&T but it shouldn't take too long now. Users can head over to Settings » About device » Software update and check if the update is available in their region.
