Outlook.com silently rejecting mail from domains lacking DMARC — authentication now effectively mandatory

Context

Microsoft's Outlook.com is reportedly rejecting inbound email from sending domains that lack a DMARC record, without announcing the change. The move effectively makes email authentication mandatory for anyone who wants reliable delivery to Outlook addresses. Email marketing and lifecycle teams, and anyone sending transactional mail from unauthenticated domains, should check their DMARC status before assuming their campaigns are landing.