tl;dr: Microsoft's consumer mail services (Outlook.com, Hotmail, Live, MSN) have quietly started rejecting some messages from domains without a DMARC record, returning a "550 5.7.1 Message lacks SPF/DKIM/DMARC Auth Records" bounce. The rejection is intermittent and hits small senders too, not just the bulk senders covered by Microsoft's published May 2025 rules. The fix is publishing a single TXT record at `_dmarc.yourdomain` with value `v=DMARC1; p=none`, assuming SPF or DKIM already aligns with your From domain.