This source is known to forward email
forwarding notes:Microsoft runs a complicated infrastructure, with email routing through and between various services on route to final delivery. For an overview from Microsoft on their forwarding challenges, see https://blogs.msdn.microsoft.com/tzink/2016/05/19/why-does-my-email-from-facebook-that-i-forward-from-my-outlook-com-account-get-rejected/ Messages reported as passing SPF in alignment will be recognized as first-party from this infrastructure.
Microsoft's Outlook infrastructure is capable of sending DMARC compliant messages for subscribed customer domains. Consult with their support for guidance on setting this up.