Microsoft has responded to Delta Air Lines’ criticism of Windows and CrowdStrike after the giant IT outage last month. Delta CEO Ed Bastian wants compensation from both CrowdStrike and Microsoft for the estimated $500 million Delta lost due to the outage. Now, Microsoft says Delta refused its free help on multiple occasions and even ignored an email from CEO Satya Nadella to Bastian.

“Microsoft empathizes with Delta and its customers regarding the impact of the CrowdStrike incident. But your letter and Delta’s public comments are incomplete, false, misleading, and damaging to Microsoft and its reputation,” says Mark Cheffo, co-chair of Dechert’s global litigation practice, in a letter on behalf of Microsoft to Delta’s lawyers.

The letter, embedded below, aims to paint a very different picture of the incident following Bastian’s comments in an interview with CNBC last week. Bastian called Microsoft fragile and asked, “When was the last time you heard of a big outage at Apple?” He also revealed that more than 40,000 of the company’s servers had been hit by CrowdStrike’s faulty update. Microsoft’s letter suggests that Delta’s problems might run a lot deeper than its Windows server outage, though.

“Even though Microsoft’s software had not caused the CrowdStrike incident, Microsoft immediately jumped in and offered to assist Delta at no charge following the July 19th outage,” says the letter from Cheffo. “Each day that followed from July 19th through July 23rd, Microsoft employees repeated their offers to help Delta. Each time, Delta turned down Microsoft’s offers to help, even though Microsoft would not have charged Delta for this assistance.”

Microsoft also claims that an employee contacted Delta on July 22nd to offer any help the airline needed, but a Delta employee replied that things were “all good” on the same day Delta canceled more than 1,100 flights, followed by 500 more cancellations the day after.

“More senior Microsoft executives also repeatedly reached out to help their counterparts at Delta, again with similar results,” writes Cheffo. “Among others, on Wednesday, July 24th, Microsoft CEO Satya Nadella emailed Delta CEO Ed Bastian, who has never replied.”

Bastian may well have missed that email from Nadella because he was busy flying to the Olympic Games in Paris, as Delta is the official airline for Team USA. Among all the flight cancellations after the CrowdStrike outage, Delta had to scramble to fulfill its Team USA obligations to get athletes to Paris on time.

Microsoft thinks Delta refused its free help because it was actually struggling to restore non-Windows systems instead. “It is rapidly becoming apparent that Delta likely refused Microsoft’s help because the IT system it was most having trouble restoring — its crew-tracking and scheduling system — was being serviced by other technology providers, such as IBM, because it runs on those providers’ systems, and not Microsoft Windows or Azure,” says Microsoft’s letter. 

That suggests that Delta was hit by the CrowdStrike outage on its Windows systems and that those failures then impacted its IT infrastructure that was serviced by IBM and others. Microsoft says Delta “apparently has not modernized its IT infrastructure,” so it was more impacted by the CrowdStrike outage than rivals like American Airlines or United Airlines.

Like CrowdStrike, Microsoft is also asking Delta to preserve documents related to the CrowdStrike outage. It also wants the airline to keep anything related to the outage of its crew-tracking and scheduling systems that run on a mixture of IBM, Oracle, Amazon Web Services, Kyndryl, and other technologies. Microsoft says it will “vigorously defend itself in any litigation if Delta chooses to pursue that path.”

Earlier this week, CrowdStrike also claimed it’s not to blame for Delta’s days-long outage and said Delta also declined its on-site assistance. CrowdStrike’s comments now make more sense after Microsoft’s suggestion that the problems at Delta could run a lot deeper than its Windows systems being taken down by the faulty CrowdStrike update. Unlike other airlines, Delta struggled to get systems back online and is currently being investigated by the US Department of Transportation over its handling of recovery efforts.