Southwest Airlines (NYSE: LUV) is one of Boeing's (NYSE: BA) most important customers. The low-fare carrier has been exclusively flying the Boeing 737 for several decades (with only brief exceptions), and now has more than 700 of them in its fleet.At Southwest's investor day last week, the carrier revealed that it has agreed with Boeing to defer 67 737 MAX deliveries scheduled for the 2019-2022 period until 2023 and beyond.