An X-header is a customer addition incorporated into the header of an email for a particular reason. Popular X-headers include subscriber-ID, company-ID, or campaign-ID.
The X in X-headers represent the experimental nature of the value or an extension of the standard header. Senders can track, add, or report data in these customized headers if they use an ESP that supports it. It is easier to manage email delivery if email metrics like bounces, sent, opened, clicked, and complaints are properly tracked and reported.