Implementing which of the following will decrease spoofed e-mail messages?

Sender Policy Framework (SPF) is an email authentication method designed to prevent sender address forgery. A domain owner publishes a special DNS record (an SPF record) that lists all the IP addresses of the mail servers authorized to send email on behalf of that domain. When an inbound mail server receives an email, it checks the SPF record of the purported sending domain. If the IP address of the sending server is not on the authorized list, the receiving server can identify the message as potentially spoofed and can subsequently reject it or flag it as spam, thereby decreasing the effectiveness of spoofed email messages.

A. Finger Protocol is an obsolete protocol used to retrieve information about users on a remote system and has no function in email authentication.

C. Network Address Translation (NAT) is a networking method for remapping IP addresses, primarily used for IP conservation and network security, not for authenticating email senders.

D. Internet Message Access Protocol (IMAP) is a standard protocol used by email clients to retrieve email messages from a mail server; it does not validate the sender's identity.

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1. Kitterman, S. (2014). Sender Policy Framework (SPF) for Authorizing Use of Domains in Email, Version 1. RFC 7208, IETF.

Section 1, Introduction: "Email on the Internet can be forged in a number of ways... This document describes the Sender Policy Framework (SPF), a mechanism by which a domain owner can publish a list of servers that are authorized to send email on its behalf. This can be used to detect and reject forged addresses in the SMTP MAIL FROM command."

2. National Institute of Standards and Technology (NIST). (2019). Special Publication 800-177 Revision 1: Trustworthy Email.

Section 3.1, Sender Policy Framework (SPF): "SPF is a widely deployed technology that allows a domain owner to authorize hosts to send mail on behalf of the domain. The domain owner publishes this authorization policy in a special DNS resource record. A receiving mail server can use the policy to determine whether a message was sent from an authorized host." (DOI: https://doi.org/10.6028/NIST.SP.800-177r1)

3. Google Workspace Admin Help. (n.d.). Help prevent spoofing & phishing with SPF. Google.

"How SPF helps prevent spoofing": "Without email authentication, spoofers can more easily send messages that appear to be from your organization... SPF is a standard email authentication method that helps prevent spoofing by letting receiving mail servers check that messages appearing to come from your domain are sent from servers authorized by you." (Retrieved from Google's official documentation for administrators).