X-headers are email headers that are added into the email in addition to the standard headers, such as the To, From, and Subject, according to the specific needs of the sender. Mailbox providers also add X-headers to email, for things such as authentication results, spam filter information, and more.
X-headers have traditionally started with an X to denote that the value is experimental or an extension of the standard header.
Some Email Service Providers (ESPs) offer the ability to add, track, and report on data included in these customized headers. Tracking and reporting on email metrics like sent, opened, clicked, bounces, and complaints are essential for managing email delivery.
Received: by 10.194.36.97 with SMTP id p1csp335466wjj;
Thu, 9 Jun 2016 06:50:18 -0700 (PDT)
X-Received: by 10.129.76.66 with SMTP id z63mr5699175ywa.136.1465480218403;
Thu, 09 Jun 2016 06:50:18 -0700 (PDT)