The Chief Information Officer of a large cloud software vendor reports that many employees are falling victim to phishing emails because they appear to come from other employees. Which of the following would BEST prevent this issue?

A. Include digital signatures on messages originating within the company.
B. Require users to authenticate to the SMTP server.
C. Implement DKIM to perform authentication that will prevent the issue.
D. Set up an email analysis solution that looks for known malicious links within the email.