What does “compauth=fail reason=010” mean?

What does it mean?

Reason 010 means the From domain is one of your organisation's accepted domains, and the message failed your DMARC policy at p=quarantine or p=reject.

What exactly does the source say?

010 The message failed DMARC, the DMARC policy action is p=reject or p=quarantine, and the sending domain is one of your organization's accepted domains (self-to-self or intra-org spoofing).

Why would a small firm see it?

A forged message aimed at your staff, or a copier or app sending in your name from outside without authentication.

How do I fix it?

1. Check who really sent it: the Authentication-Results header shows the sending server.
2. If it is one of your services, add it inside your one SPF record and turn on its DKIM signing.
3. If it is not, the policy did its job.Warn staff about the message.

What else do the sources say?

Composite authentication result. Microsoft 365 combines multiple types of authentication (SPF, DKIM, and DMARC) and other parts of the message to determine whether the message is authenticated. Uses the From: domain as the basis of evaluation. Note: Despite a compauth failure, the message might still be allowed if other assessments don't indicate a suspicious nature.

dmarc=fail action=oreject header.from=o365e.onmicrosoft.com;compauth=fail reason=000