Do we not meet High Volume Sender authentication requirements?

The annual number of notifications per recipient is not enough to establish whether the sending domain falls under Microsoft's high-volume requirements. First obtain the exact NDR: the documented 550 5.7.515 error specifically identifies insufficient sender-domain authentication. If that is the code, verify actual messages from your production sending service, not only the existence of DNS records. Microsoft requires SPF and DKIM to pass, a published DMARC policy, and DMARC alignment through at least one of SPF or DKIM with the visible From domain. A third-party sender can publish valid records yet send using an unexpected envelope or signing domain. Capture the sending IP, envelope sender, visible From and DKIM signing domain, together with authentication results from a delivered test. Share sanitized headers and the complete bounce text. If the error code differs, investigate that code rather than assuming this policy explains every rejection.