X-header

User-defined, unofficial header field that exists in the message header. X-headers are not specifically mentioned in RFC 2822, but the use of an undefined header field starting with "X-" has become an accepted way to add unofficial header fields to a message. Messaging applications, such as anti-spam, antivirus, and messaging server applications may add their own X-headers to a message. X-header fields are usually preserved but ignored by messaging servers and clients that don't use them. [Exchange 2007]