How to identify suspicious MX records and what tools to use for checking them?

Key findings

• Absence or timeout: A domain with no MX records or one that consistently times out during a DNS query suggests that it cannot receive email. This could indicate an unmaintained domain or a deliberate attempt to avoid email. Identifying suspicious email domains is key.
• Unusual priority values: MX records have priority numbers (lower is preferred). Unusual or excessively high priority values might indicate a misconfiguration or an attempt to route mail through unexpected servers.
• Generic hostnames or IP addresses: While some legitimate services use generic hostnames, a complete lack of identifiable branding or an MX record pointing directly to an IP address can sometimes be suspicious, especially if it's not a known mail provider.
• Mismatch with domain history: If a domain suddenly changes its MX records to an obscure or unknown mail server, it could be a sign of compromise or a new, potentially less reputable, email service provider.

Key considerations

• Use reliable lookup tools: Tools that perform thorough DNS checks are more reliable than basic command-line utilities. Consider using services like NsLookup.io for comprehensive MX data.
• Cross-reference with blacklists: Check if the IP addresses associated with the MX records are present on any major blocklists or blacklists. This is a strong indicator of potential abuse.
• Verify related DNS records: Suspicious MX records often go hand-in-hand with issues in SPF, DKIM, and DMARC records, which are vital for email authentication.
• Understand timeouts: A DNS timeout doesn't always mean the records don't exist; it simply means no response was received. Further investigation with different tools or at a later time is often necessary.

Key opinions

• Experience is vital: Beyond basic checks, understanding what looks suspicious in MX records often comes from knowing typical configurations for various email providers.
• Absence of records: The most straightforward sign of a problematic MX record is its complete absence, meaning the domain cannot receive mail.
• DNS timeouts: A DNS timeout during an MX lookup can indicate that mail wouldn't be delivered, but it doesn't necessarily mean the record doesn't exist. Further investigation is needed.
• Tool limitations: Some common tools, like nslookup, can provide misleading diagnostics, making it important to use more robust alternatives for accuracy.

Key considerations

• Automated vs. Manual: While automated tools are great for bulk checking and cleaning MX records, manual checks with advanced tools like dig provide deeper insights.
• Provider identification: Knowing how to identify the SMTP provider from an MX record helps assess legitimacy.
• Email flow implications: Marketers must understand that suspicious MX records directly impact email delivery rates, potentially leading to hard bounces or being flagged as spam.
• Community insights: Engaging with communities like Spiceworks Community provides practical perspectives on MX record troubleshooting.

Key opinions

• Holistic view: Experts agree that suspicious MX records are rarely isolated issues. They often correlate with other DNS anomalies, particularly if email authentication records are missing or misconfigured.
• Reputation is key: The reputation of the mail server identified by the MX record is paramount. Checking its IP and domain against public blocklists and threat intelligence feeds is a standard practice.
• Dynamic DNS analysis: Suspicious activity can involve frequent, unexplained changes to MX records, suggesting domain hijacking or attempts to bypass security measures.
• Spam trap indicators: Some MX records (or their absence) can be strong indicators of spam traps, especially for domains that have been dormant or explicitly set up to catch spammers.

Key considerations

• Beyond simple lookups: While basic MX lookup tools are a starting point, experts advise using comprehensive DNS analysis platforms that can also perform sender reputation checks.
• Blocklist integration: Automated tools that cross-reference MX record IPs with a wide range of email blocklists (or blacklists) are essential for real-time monitoring.
• Domain registration analysis: Examining the domain's registration details (WHOIS) can sometimes reveal inconsistencies or suspicious ownership that aligns with problematic MX records.
• Threat intelligence feeds: Subscribing to and integrating with threat intelligence feeds provides early warnings about newly identified malicious domains or IPs, including those used in MX records.

Key findings

• RFC 5321 (SMTP) requirements: This RFC defines that an MX record must point to a valid hostname (A or AAAA record) that resolves to an IP address. Failure to resolve indicates a non-functional mail exchanger.
• Priority values: RFCs specify that lower priority values are preferred. Unusual or identical priority numbers across multiple records, especially without proper mail server setup, can lead to unpredictable mail delivery.
• No MX record: If no MX records are found for a domain, mail servers should fall back to an A record. However, this is discouraged for active mail reception and can indicate a domain not set up to receive email properly.
• DNSSEC validation: DNSSEC-signed zones provide cryptographic authentication for DNS data. An MX record in a non-DNSSEC validated zone, or one that fails validation, could be more susceptible to tampering.

Key considerations

• Adherence to RFCs: Understanding what RFCs mandate for email helps identify non-standard or potentially problematic MX setups.
• Impact on deliverability: Misconfigured or absent MX records are fundamental barriers to email delivery. Ensuring proper configuration is a basic step to boosting deliverability rates.
• DNS query tools: Documentation often recommends specific DNS query tools (e.g., dig) over others (e.g., nslookup) due to their advanced features and more accurate reporting for DNS records.
• Error codes and responses: Understanding standard DNS error codes and responses, such as SERVFAIL or NXDOMAIN, is critical for diagnosing MX record issues from a technical perspective.