You may not have flown on Air Serbia, but it is a legitimate airline with landing privileges in the United States. They are offering a crazy-low $227.06 round-trip fare from New York JFK to Sophia, Bulgaria this Fall and Winter.
UPDATE: Now $143 to Belgrade, as per http://viewfromthewing.boardingarea.com/2016/08/14/unbelievable-airfare-visit-eastern-europe-143-roundtrip-new-york-jfk-belgrade/
Bulgaria is an Eastern European nation with a long history and many things to see and do. The nation is friendly to the United States and considered safe to visit.
The FAA monitors airlines that fly within and to the United States, and will not permit those that do not meet minimal safety standards.