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^[1]:
_dmarc.example.com
and this value
v=DMARC1; p=none; rua=mailto:postmaster@example.com;

try http://kitterman.com/dmarc/assistant.html

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 their video about DMARC benefits

Tools

dig any _dmarc.example.com +trace

References

↑ Namecheap has a particular quirk where the 'label' (or host) portion of the TXT record should just be '_dmarc' and their system will append the domain for you.

[1] Namecheap has a particular quirk where the 'label' (or host) portion of the TXT record should just be '_dmarc' and their system will append the domain for you.

[1]