Nick wants to allow email servers to validate that email from his servers is actually from them. What email security framework should he adopt to allow this?

DKIM (DomainKeys Identified Mail) validates a domain’s identity using a public key pair, validating the authenticity of the sender. DMARC (Domain-based Message Authentication, Reporting, and Conformance) controls how unauthenticated messages are handled by mailbox providers, including quarantining, rejecting, or rejecting messages. The Sender Policy Framework (SPF) lists IP addresses of systems allowed to send email in DNS TXT records for a domain. STP is not an email security framework. The Simple Mail Transfer Protocol (SMTP) is the default email protocol.