DMARC
From Freephile Wiki
TLDR[edit]
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[edit]
IETF Specification[edit]
https://tools.ietf.org/html/rfc7489
Vendors[edit]
https://dmarcian.com/ is a place where you can get your reports nicely aggregated and reported.
See Also[edit]
Add a DMARC record in Google support
