CHICAGO (Reuters) - Microsoft ( MSFT ) blamed Delta Air Lines ( DAL ) on Tuesday for its dayslong struggle to recover from a global cyber outage that led it to cancel more than 6,000 flights.

The flight disruptions stranded hundreds of thousands of travelers and are estimated to cost the Atlanta-based airline $500 million.

Last week, Delta's CEO Ed Bastian told CNBC that while the airline heavily relies on Microsoft ( MSFT ) and CrowdStrike ( CRWD ), both tech companies had failed to deliver an "exceptional service." In the interview, he also said that Microsoft ( MSFT ) had the "most fragile platform."

Microsoft ( MSFT ) said it is still investigating the circumstances surrounding the CrowdStrike ( CRWD ) incident to understand why other airlines, including American and United, fully restored their operations so much faster than Delta.

It, however, added that its preliminary review suggested that Delta, unlike its competitors, apparently had not modernized its IT infrastructure.

In a letter from an external lawyer, the company dubbed the airline's comments "incomplete, false, misleading, and damaging to Microsoft ( MSFT ) and its reputation."

Microsoft ( MSFT ) said its employees repeatedly offered to help Delta following the outage, but the U.S. carrier turned them down. The tech giant said its CEO Satya Nadella emailed Bastian, but never got a reply.

It accused Delta of using services of other technology providers for its crew-tracking and scheduling system and alleged it as a likely reason for declining its help.

Delta was not immediately available for comments.

CrowdStrike ( CRWD ) has also rejected Delta's claim that it should be blamed for flight disruptions.