The bottom line is, if you are planning to fly in the United States, you are going to need a face mask. Every major U.S. airline now requires them, and some even require that you have them on during the entirety of your journey – starting at check-in and ending when you pick up your bags at baggage claim and leave the airport.
Delta Air Lines is particularly strict with its face mask rules. Any traveler who claims that they cannot use a mask for medical reasons must be pre-cleared by the airline via a 'Clearance-To-Fly' process. To board a flight, a basic cloth covering the face, including the mouth and nose, works, and you can even use a plastic face shield, but wearing a face shield doesn't exempt you from flying without a mask.
To protect passengers and crew, the airline recently updated its mask policy. The updated policy does not allow travelers to use any mask with an exhaust valve. Masks with exhaust valves actually have nearly the opposite effect and could endanger other travelers as the exhaust valve allow droplets out into the cabin.
If you plan to fly Delta anytime soon, remember that you need to have a face mask on when you check in, when you visit Delta Sky Clubs, while at the boarding gate, on the the jet bridge, and of course, you must have your mask on at all times while on the plane, with meal service and general drinking and eating being the only exceptions. The airline also strongly recommends that you use a mask in high-traffic areas at the airport such as in the bathroom and when waiting in security lines.