X- Headers

What are X- Headers?

X-Headers are custom email headers that begin with the prefix “X-” and are used to include additional, non-standard information in an email message. These headers allow mail systems, applications, or administrators to add metadata for diagnostic, analytical, or tracking purposes without interfering with standard SMTP operations. While not defined by official RFCs, X-Headers have become a common way to pass proprietary or experimental data between systems.

Examples of X-Headers include internal routing tags, anti-spam results, and mail-handling identifiers. For instance, mail gateways often add headers such as X-Spam-Status, X-Originating-IP, or X-Mailer to record how a message was processed or where it originated.

How X-Headers Work

X-Headers follow the same structural format as standard email headers, consisting of a name, colon, and value. They are inserted by mail servers or software as messages pass through the delivery chain. Each additional header is appended to the message and preserved during transmission unless explicitly removed by downstream systems.

Example of a message containing X-Headers:

These headers are primarily informational. However, some mail security and filtering tools rely on them for reporting, routing decisions, or correlation with logs. Because they are not standardized, interpretation may vary depending on the product or provider that generates them.

Common Uses of X-Headers

X-Headers are used across different stages of the mail lifecycle for troubleshooting, analytics, and custom workflows. Common categories include:

• Diagnostic data: Tracking message routing paths or delivery times
• Security context: Adding spam and malware scan results (e.g., X-Spam-Flag, X-Virus-Scanned)
• Application information: Identifying sending software (X-Mailer or X-Sender)
• Internal tracking: Recording message IDs for CRM or ticketing systems
• Authentication reporting: Supplementing SPF, DKIM, or DMARC evaluations

Some mailbox providers and anti-abuse systems also insert proprietary X-Headers to mark verified, suspicious, or quarantined messages. For example, Microsoft’s mail servers often include X-MS-Exchange-Organization-AuthAs or X-Forefront-Antispam-Report headers to convey internal authentication and spam filter results.

Security and Privacy Considerations

Although X-Headers are useful for diagnostics, they can inadvertently expose sensitive information if not managed carefully. Headers revealing internal IPs, usernames, or system details can aid attackers in reconnaissance. For this reason, many organizations strip unnecessary X-Headers from outgoing mail before delivery.

Best practices for handling X-Headers include:

• Remove or anonymize internal IPs and hostnames before sending external mail
• Standardize header usage across servers and applications
• Document custom headers to avoid conflicts with vendor-generated ones
• Monitor for unexpected or malicious X-Headers that might indicate tampering
• Implement data loss prevention (DLP) policies to restrict sensitive metadata exposure

As industry standards evolve, the use of “X-” prefixes is being replaced by registered custom header fields under vendor namespaces, but many systems still rely on legacy X-Headers for backward compatibility.

X-Headers and DMARCeye

DMARCeye analyzes X-Headers as part of its email inspection and forensic reporting processes. By examining headers added by mail servers, security tools, and relay systems, DMARCeye helps identify how messages were authenticated, routed, and handled across different networks.

The platform correlates X-Header data with DMARC forensic reports and authentication results to detect anomalies, unauthorized relays, or policy misconfigurations. This insight helps administrators trace the full journey of each email, from sending server to recipient inbox, improving visibility and strengthening domain protection.

Sign up for a free trial of DMARCeye today and secure your email domain.

To learn more about DMARC and DMARC-related terms, explore the DMARCeye Glossary.