Will my emailed ticket/receipt be sufficient to board my domestic flight? I do not own a Smartphone. Delta has a two-hour wait time for customer service.
It's sufficient to check-in but not to board the plane.
Your ticket/receipt includes a reservation/booking number or PNR (six letter/numbers). Starting 24-ish hours before departure you can use that to check in either online or at a kiosk at the airport. That's when you get the actual boarding pass (print it at home or at the kiosk). You need the boarding pass and a photo-ID to get through security and on the plane.
CAVEAT: there may be additional Covid related steps you need to take depending on the details of your flight.