Which of the following is NOT an email authentication standard to control domain sending authority?

The correct answer identifies SMTP as not being an email authentication standard aimed at controlling domain sending authority. SMTP, or Simple Mail Transfer Protocol, is primarily a protocol used for sending and receiving emails over the Internet. It defines the rules for how emails are transmitted but does not provide mechanisms for verifying whether the sender is authorized to send emails on behalf of a domain.

In contrast, SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting & Conformance) are all standards specifically designed to help authenticate the sender's domain and prevent email spoofing.

SPF allows domain owners to specify which IP addresses are permitted to send emails on behalf of their domain, thus controlling sending authority. DKIM involves the signing of emails with a cryptographic key, ensuring that the email hasn’t been altered and verifying the domain of the sender. DMARC builds upon SPF and DKIM by allowing domain owners to publish policies on how to handle emails that fail authentication checks, thus further enhancing domain sending authority control.

Therefore, while SPF, DKIM, and DMARC are foundational standards for establishing trust in email communications and protecting against unauthorized use of domains, SMTP’s role is strictly to facilitate the transmission of messages,