Mail to Outlook.com now bounces with:
550 5.7.1 Message rejected: Message lacks SPF/DKIM/DMARC Auth Records
No official announcement yet, and it hits low-volume senders too. The fix is _dmarc record:
_dmarc TXT "v=DMARC1; p=none; rua=mailto:[example.com]@rua.dmarcdkim.io"
Full article: https://dmarcdkim.com/blog/outlook-message-lacks-spf-dkim-dmarc-auth-records