The airline said it will address the situation with the pilot and that it doesn't condone employees sharing personal political opinions while working or tolerate behavior that leads to divisiveness.
Hundreds of Southwest Airlines flights are either canceled or delayed on Monday, after a weekend of disruptions that the carrier has blamed on air traffic control issues and weather conditions.
The airlines will require all customers over the age of 2 to wear face coverings on board and at airports, with no exceptions. They say people who don't comply won't be allowed to fly as next week.
One pilot messaged that the problem was "egregious" and he had "basically lied to the regulators (unknowingly)" when he had told the FAA that the flight control system was safe.
The pilots say they stand to lose $100 million in income. Southwest has canceled more than 30,000 flights since the FAA grounded the aircraft in March after two crashes killed 346 people.
The FAA says the issue that affected Southwest, Delta and other carriers has been resolved. But with hundreds of flights affected, the delays could linger.
Some U.S. airlines are scrambling to rebook passengers after the FAA announced it was grounding the Boeing 737 Max fleet. Airlines affected include Southwest, American and United.