ESA's are supposed to be documented and have the proper paperwork. Airlines most definitely can ask to see the paperwork and deny the animal if the passenger does not have the proper paperwork. The restriction on the airlines is they cannot ask what the medical condition is. Glad to see Delta is going to start enforcing. Wish all the airlines would start enforcing. It can only help the airlines bottom line, more revenue for the pets, and the other passengers, no more fake ESA animals that can cause problems. I hav nothing against people flying with their pets, but they need to follow the rules like everyone else. No more free rides.