DMARC record
DMARC is a system that adds some rules for DKIM and SPF, so remote servers have a better idea of what your intention is for messages that fail those 2 systems.
DMARC requires both the SPF and DKIM to be present in order to work.
Contents
SPF - Sender Policy Framework
Anything that contains the word "framework" is typically crap. Here's more crap we're stuck with.
SPF allows administrators to specify which hosts are allowed to send email from a given domain by creating a specific DNS SPF record in the public DNS.
Mail Exchanger (MX records) then uses the DNS to check that mail from a given domain is being sent by a host sanctioned by that domain's administrators.
DKIM - Domain Key Identified Mail
DKIM uses a key pair, consisting of a public key and a private key. Again more confusing and overly complicated crap!
DMARC - Domain-based Message Authentication, Reporting and Conformance
A DMARC policy allows a sender to indicate that its emails are protected by SPF and/or DKIM, and tells a receiver what to do if neither of those authentication methods passes.
references
To add a DMARC record, go to your domain's DNS Management,
