Using New Outlook on Windows for personal email, calendar, and contact management
It has to be Yahoo targeting Microsoft. Even though Yahoo states in their latest documentation that for normal users only need SPF *or* DKIM clearly that's not working with Microsoft custom domain accounts.
I've been able to successfully send mail from in-house email servers using an SMTP service using only SPF to Yahoo so it's not clear why they're targeting Microsoft requiring DKIM.
Microsoft finally responded this morning to the ticket I opened on Saturday. By then I had already found the answer. I explained to the agent that adding a DKIM record solved the problem, this was the agent's response:
..Your insight and experience will be valuable for other customers who may encounter similar issues. I will proceed with archiving this service request and share the information with the team as a possible resolution..."
What's worse is that his first response to the issue (before I told him about DKIM) was to tell me to contact Yahoo.