Email authentication (or email validation) is a collection of protocols used to verify the source and legitimacy of email messages.
SPF (Sender Policy Framework) is a form of email sender authentication. It validates if an email was sent from an authorized IP address. An SPF record is automatically added to every domain.
DKIM (DomainKeys Identified Mail) is a form of domain authentication that signs the message and verifies it was sent from the source domain. DKIM can be enabled on a per domain basis.
To enable DKIM on a domain: