Using Classic Outlook on Windows for personal email, calendar, and contact management
This is correct, it's a DNS setup issue. SPF / DKIM / DMARC are moving to being REQUIRED now.
Everyone needs to get comfortable with these adjustments on their DNS provider. EX711592 calls this out directly.
"If your admins have already confirmed that your organization's DKIM, SPF, and DMARC records have been established already, and your users haven't received the error code mentioned below in NDRs, then you can safely ignore this message. "
There are things behind the scenes being implemented by Google, AOL, Yahoo - and they are a hard requirement now.
Now then, those 3 things can each be done properly or improperly, which is another troubleshooting issue entirely.