OP - There is no such thing as a layover. You are either traveling on a single ticket from your origination airport to ATL and connecting to a flight to MAD or you are on two tickets and you are transfering between those two tickets at ATL.
If the former, this is apparently within the Minimum Connection Time (MCT) for your carrier(s), but it is outside what I consider to be the Sensible Connection Time (SCT). DL requires that you be at your departure gate for MAD no later than 30 minutes prior to departure and it considers the arrival time of your inbound flight to ATL to be when the Captain locks the brakes at the gate on arrival.
Thus, after the brakes are locked, the jet bridge is attached, and you make it from your inbound aircraft and head for your departure gate, you have a total of 14 minutes.
While DL will not generally cancel your ticket at T-30, it reserves the right to do so and will offload you if the flight is overbooked and at risk of oversale.
Unless you have critical meetings on both sides of your trip, there is no reason to book a connection such as this.
If you are on two tickets, you have no "protection." This means that if you are late for your MAD flight, DL simply treats you as though you skipped your flight and cancels your ticket. You then have whatever value your fare rules provide to purchase a new ticket at whatever the prices are at the time unless someone at DL takes pity on you.
As a general rule, I would not schedule less than 4 hours between tickets on an international transfer and I would prefer an overnight, given the financial risks.