X-header
A non-standard header field whose name starts with X-, such as X-Mailer, added by software for its own purposes; RFC 6648 discourages the prefix for new fields.
What it means
RFC 5322 allows any field name it does not define, and for decades the convention was to start private or experimental names with X- so they could not clash with standard ones. Sending platforms use such fields for campaign and customer IDs and routing data, and receivers add verdicts of their own, as Microsoft does with X-Forefront-Antispam-Report.
RFC 6648 (2012) recommends against the X- prefix for new fields, because experimental names often became permanent and could then never be renamed; existing X- fields stay in use. Anything in them is visible to whoever opens the header, so an X-Mailer naming a bulk tool or an internal ID tells recipients and their filters more than the sender may intend.
Example
X-Mailer: ExampleMailer 4.2Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 6648: Deprecating the "X-" Prefix and Similar Constructs in Application Protocolsrfc-editor.org
- RFC 5322, section 3.6.8: Optional Fieldsrfc-editor.org
- Microsoft Learn: Anti-spam message headers in Microsoft 365learn.microsoft.com
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