NSA probably. Gives them plausible deniability.

Maybe some of their targets did use example.com for some probing, and the NSA had a hand in Sumitomo Electric Industries' mail server.

Reading the article, there is a huge flaw in the autodiscover protocol by Microsoft.

https://www.akamai.com/blog/security/autodiscovering-the-gre...

According to it, it seems that if someone registers autodiscover.com then example.com lacking autodiscover.example.com will make Outlook try checking if autodiscover.com has an entry.

It's just a braindead system.