Since roughly November 5, Outlook Hotmail, Live and MSN are rejecting some mail from domains without DMARC records.
This includes low volume domains with passing SPF and DKIM results.
The specific bounce we are seeing is "550 5.7.1 Message rejected: Message lacks SPF/DKIM/DMARC Auth Records".
This is a different outcome from the high volume sender rule (5.7.515) which is part of why this looks like an unannounced extension. I haven't seen anything from Microsoft confirming any change.
The current fix is minimal - just add a DMARC record of something like `v=DMARC1; p=none;` if a domain already has a record though, leave it alone.