We are currently seeing increased rejection rates for emails sent to Microsoft-hosted addresses (Outlook, Hotmail, Live, MSN).
This issue is related to an ongoing problem acknowledged by Microsoft, where certain sending IPs are being temporarily rejected at higher rates. Microsoft is actively investigating. More info: https://substrate.office.com/ip-domain-management-snds/Postmaster
Delivery to non-Microsoft providers is unaffected. We are monitoring the situation and will provide updates as more information becomes available.