An X-header is a custom, non-standard email header, conventionally prefixed with X-, used to carry extra metadata such as campaign or system information alongside a message.
Category
Deliverability
Also known as
custom email header; X- header
X-headers are a practical diagnostic tool in deliverability work: they can tag which system sent a message, carry campaign identifiers, and help trace a message’s path when inboxing goes wrong. They sit alongside the authentication headers that DMARC and SPF rely on.
In practice
Use X-headers to make messages traceable, not to fix deliverability. They help you diagnose which stream or campaign a problem sits in, but reputation and authentication, not custom headers, decide whether mail reaches the inbox. Do not put personal or sensitive data in them, because headers can be exposed through forwarding, logs, or support tooling.
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