Authentication-Results lets an authentication service record results such as SPF, DKIM and DMARC in a structured header.

A simplified example:

Authentication-Results: mx.example.net;
  spf=pass smtp.mailfrom=sender.example;
  dkim=pass header.d=example.com;
  dmarc=pass header.from=example.com

This is extremely useful during troubleshooting because it shows both the result and the identity evaluated.

Do not trust every copy

A sender can inject a fake Authentication-Results field before transmission. Receivers need a defined trust boundary and should remove or distinguish untrusted copies.

Reference material