One hour should be sufficient unless there are weather or other delays. ATL has an underground tram system that is very efficient once you get to it. Chances are you will depart for SXM from a terminal that is different than your arrival terminal into ATL. There is also usually 20 minutes or so of extra time built into most flights so you have that to work with also.

While one hour is considered a "legal" connection, it has been mentioned on here multiple times that airlines sometimes do end up with connections that are not "legal" due to schedule changes, people not paying attention to the times or just their own scheduling.

Good luck.