X-headers are custom email headers added into 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 authentication results, spam filter information, and more.
X-headers start 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 custom email headers. Tracking and reporting on email metrics like sent, opened, clicked, bounces, and complaints are essential for managing email delivery.
For example:
Delivered-To: example@returnpath.com
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)
