Why this error is more specific than a generic DMARC failure
Microsoft documents 550 5.7.515 for high-volume senders whose visible From domain does not meet the required authentication level for Outlook.com and related consumer services. The NDR can include SPF, DKIM and DMARC observations. Preserve those observations before making changes.
Validate SPF and DKIM separately
For the affected high-volume path, Microsoft expects both SPF and DKIM requirements to be met, while DMARC validation also needs an aligned path. That means a message can present a DMARC pass but still require attention if one of the underlying authentication mechanisms does not meet the provider’s high-volume requirement.
Alignment still matters
The domain in the Return-Path and the DKIM signing domain can differ from the visible From domain. DMARC only uses an authentication path when the relevant domain aligns with the From identity. For third-party senders, custom return-path and DKIM domain configuration are therefore central to a durable correction.
Use the exact production sender for the verification
Testing from an employee mailbox does not prove that a marketing ESP or transactional API is configured correctly. Reproduce the same sending service and From domain that generated the NDR, then inspect the actual delivered or rejected message evidence.
Verify the evidence
Use the Email Infrastructure Digital Twin to establish the public control plane, then inspect message-level evidence with Email Header Forensics.