DMARC

TLDR

You need to have SPF and DKIM properly established first.

(assuming your domain is 'example.com') Publish a DNS TXT record with this label:
_dmarc.example.com
and this value
v=DMARC1; p=none; rua=mailto:postmaster@example.com;

Background

https://dmarc.org/

IETF Specification

https://tools.ietf.org/html/rfc7489

Vendors

https://dmarcian.com/ is a place where you can get your reports nicely aggregated and reported.

See Also

Add a DMARC record in Google support

Domain Name System. An Named Host to IP Address lookup system

Internet Engineering Task Force = organization to make the Internet work better