Delta Air Lines canceled flights for a third straight day Monday, as it struggles to recover from a global software outage that took down Microsoft systems around the world. The global outage, related to a faulty update from cybersecurity company CrowdStrike, also hindered operations at shipping and logistics companies, retailers and banks.

Delta CEO Ed Bastian said in a public letter Sunday that it paused flying after the outage occurred on Friday, resulting in 3,500 canceled flights through Saturday, and continuing through Sunday. The outage hit on what was the airline's "busiest travel weekend of the summer," according to Bastian.

On Monday, when most other airlines had largely recovered from the software outage, another 1,120 Delta flights within, into or out of the U.S. were canceled or delayed, according to flight tracking website FlightAware. In comparison, United Airlines canceled 17 flights, while another 112 of the carrier's flights were delayed.

Bastian said many Delta applications run on Microsoft Windows, which was affected by the CrowdStrike issue. Most crucially, one of the airline's crew tracking tools was rendered inoperable, making it impossible for Delta to "effectively process the unprecedented number of changes triggered by the system shutdown," Bastian said. "Our teams have been working around the clock to recover and restore full functionality."

Bastian said Delta would offer travel waivers to customers on affected flights, allowing them to change their itineraries and rebook flights without a fee. "I encourage you to take advantage of that flexibility if possible," Bastian said in his letter. 

Secretary of Transportation Pete Buttigieg took aim at Delta on X, calling the way the airline was treating customers during the business interruption "unacceptable."

"We have received reports of continued disruptions and unacceptable customer service conditions at Delta Air Lines, including hundreds of complaints filed with @USDOT. I have made clear to Delta that we will hold them to all applicable passenger protections," he wrote.

He also made clear that under federal regulations, customers are not obligated to accept travel credit to rebook their flights, rather they are entitled to immediate refunds. 

"Delta must provide prompt refunds to consumers who choose not to take rebooking, free rebooking for those who do, and timely reimbursements for food and hotel stays to consumers affected by these delays and cancellations, as well as adequate customer service assistance," Buttigieg said.