From the original report on The Information:

For more than a year, Microsoft’s engineers and researchers have worked to create personalized AI tools for composing emails and documents by applying OpenAI’s machine-learning models to customers’ private data, said another person with direct knowledge of the plan

https://www.theinformation.com/articles/ghost-writer-microso...

(I have submitted also the original link in a separate thread, but was not able to add comments for some reason; @dang you might want to merge them.)