What the source announces
For domains sending more than 5,000 messages per day to Outlook.com consumer services, Microsoft requires SPF and DKIM to pass, DMARC to be present, and DMARC alignment through SPF or DKIM.
Why it matters operationally
Teams must identify every sending source before enforcement exposes an unaligned application. Microsoft documents rejection code 550 5.7.515 for the authentication requirement.