What Is MXToolbox and How Can You Use It Securely?

**Executive Summary:** MXToolbox is a public diagnostic tool for DNS and email infrastructure (MX, A, TXT records, SMTP diagnostics, blacklist checks, and header analysis); this report outlines how defenders use it to verify SPF/DKIM/DMARC and monitor vendor email health, warns that attackers can leverage the same capabilities for reconnaissance and phishing preparation, and recommends maintaining email authentication, setting alerts, and integrating continuous monitoring (e.g., SecurityScorecard SCDR) to reduce email- and supply-chain-related risks.