Microsoft are rejecting your mail
Summary
The piece describes a Microsoft mail deliverability issue where messages with valid SPF and DKIM were rejected due to a missing DMARC policy. It explains DMARC policies and notes that Microsoft began rejecting non-DMARC mail, while accepting mail with p=none, an unannounced change. It concludes with practical steps to avoid deliverability problems, including publishing a DMARC policy (even a none policy) and reviewing DNS/email configurations.