What Is MXToolbox and How Can You Use It Securely?

​​What Is MXToolbox?

MXToolbox is a free online tool widely used by IT and cybersecurity professionals to troubleshoot email infrastructure and check domain health. It offers real-time insights into DNS configurations, email deliverability issues, blacklist status, and mail server availability.

Although it’s primarily used for diagnostics, MXToolbox also provides critical signals that can uncover misconfigurations attackers might exploit. For organizations managing external attack surfaces or vendor risk, it’s a foundational part of the open-source intelligence toolkit. Attackers can use the same tools as defenders. MXToolbox is one such resource—and your security team should know exactly what it reveals to stay one step ahead.

What MXToolbox Can Do

MXToolbox supports a wide range of functions, but its core capabilities fall into five categories:

• DNS Record Lookup: Retrieve and validate DNS entries, including MX (mail exchanger), A (address), and TXT (SPF, DMARC).
• SMTP Diagnostics: This test will connect to a mail server via SMTP and perform a simple Open Relay Test.
• Blacklist Check: Scan your IP address or domain across more than 100 DNS-based blacklists (DNSBLs) to assess whether your email might be blocked or flagged as spam.
• Email Header Analysis: This tool will translate email headers into a human readable format and analyze the results and path of the email, which can provide informative data on anti-spam results and more.
• Monitoring and Alerts: Set up alerts for when domains get blacklisted and more.

Why Cybersecurity Teams Use MXToolbox

MXToolbox helps defenders verify that their email and DNS configurations follow best practices. It’s especially valuable for:

• Checking SPF, DKIM, and DMARC records for errors or misalignment
• Diagnosing bounce-back messages
• Verifying whether third-party vendors are sending email from blacklisted IPs

How Threat Actors Use MXToolbox

Because MXToolbox is publicly accessible, attackers can use it to scan target domains before launching spoofing or phishing campaigns.

Threat actors can rely on tools like MXToolbox to:

• Test whether a domain lacks SPF, DKIM, or DMARC
• Identify exploitable open relays or improperly configured servers
• Check if their spoofed domains are currently blacklisted
• Assess how their fake messages might be routed or flagged

For this reason, organizations should proactively harden DNS records and monitor who queries their infrastructure—especially when standing up new email services or domains.

Best Practices for Using MXToolbox Securely

To get the most value from MXToolbox while minimizing risk:

• Don’t treat its output as exhaustive: MXToolbox is a useful indicator but not a replacement for vulnerability scanning or email security platforms.
• Use alerts to track third-party changes: Set up notifications for when vendors’ email servers get blacklisted or their SPF records break.
• Keep SPF, DKIM, and DMARC records up to date: Review your records regularly to ensure mail authentication stays aligned.
• Pair it with threat intelligence tools: Use platforms like SecurityScorecard to enrich MXToolbox results with breach signals, credential leaks, and domain impersonation attempts.

Integration With SecurityScorecard Use Cases

While MXToolbox offers point-in-time visibility, SecurityScorecard provides continuous monitoring of your domain and vendor infrastructure. SecurityScorecard can help pinpoint specific areas of vulnerability in email sending infrastructure, as it checks for TLS connections and more:

• SPF/DKIM/DMARC misconfigurations
• Exposed SMTP servers
• Unencrypted email endpoints
• Vendor domains with known phishing or spoofing activity

Supply Chain Detection and Response (SCDR) integrates threat actor indicators of compromise (IOCs), TLS cipher data, and email system scans to deliver a comprehensive view of domain health across your third-party ecosystem.

Final Thoughts

MXToolbox helps organizations verify basic email infrastructure hygiene and identify common misconfigurations. While it doesn’t stop attacks on its own, it provides critical visibility into the building blocks of secure communication that underpins brands and trust in email infrastructure.

In a threat landscape where email spoofing and phishing remain top attack vectors, checking your SPF, DKIM, and DMARC settings is an essential part of thorough security programs.

SecurityScorecard complements MXToolbox by continuously monitoring your external attack surface and detecting broader signals of compromise tied to email exposure, vendor weakness, and malware infrastructure.

Protect Your Supply Chain with Real-Time Threat Detection

SecurityScorecard’s SCDR solution offers continuous monitoring of your third-party ecosystem, enabling swift identification and mitigation of cyber threats. Enhance your organization’s resilience by proactively managing supply chain risks.

🔗 Understand SCDR