Employees from the Satya Nadella-led Microsoft reached out to Delta every day between July 19 through July 23 to provide technical support, and "each time Delta turned down Microsoft's offers to help," Microsoft lawyers wrote in a letter obtained by multiple media outlets.
Additionally, Nadella personally reached out to Delta Chief Executive Ed Bastian and never heard back.
"Even though Microsoft's software had not caused the CrowdStrike incident, Microsoft immediately jumped in and offered to assist Delta at no charge," the letter, signed by Mark Cheffo of Dechert LLP, added.
Delta did not immediately respond to a request for comment from Seeking Alpha.
Bastian said earlier this month that Delta would see an impact of roughly $500M from the outage, including lost revenue and "tens of millions of dollars per day in compensation and hotels."
Delta plans to seek damages from CrowdStrike and Microsoft for the outage.
Other airlines recovered from the outage faster than Delta, as it relies on Microsoft software for matching flight crews to planes. The system was unable to process the sheer volume of changes triggered by the outage, leading to more delays.
On Monday, CrowdStrike rejected Delta's claims that the cybersecurity firm was to blame for the carrier's extended flight disruptions in the aftermath of last month's global tech outage.
"CrowdStrike (CRWD) reiterates its apology to Delta (DAL), its employees, and its customers," its external lawyer Michael Carlinsky wrote in a letter to Delta's counsel David Boies.
"However, CrowdStrike is highly disappointed by Delta's suggestion that it acted inappropriately and strongly rejects any allegation that it was grossly negligent or committed willful misconduct."
Carlinsky added that CrowdStrike reached out to Delta within hours of the incident to offer assistance, after which CEO George Kurtz personally reached out to Delta's chief Ed Bastian, but received no response.
The Department of Transportation is investigating Delta's widespread flight disruptions and customer service failures.