Using older versions of Outlook for Mac in business environments
I think if you have your domain (www.mysite.com) hosted with one provider, and your email on O365 (and your DNS with another provider, perhaps), Outlook is trying to use your domain DNS record to validate your account (and not your MX record in DNS, which would point to O365).
Just a guess, but I get the same thing. I can't use 15.9 at all as every exchange account (O365 or otherwise) causes Outlook to crash. Yes I've tried the terminal commands - no dice.