If you want to define your own header fields, the specification requires that you start them with "X-" to avoid any possible confusion with previously defined or other header fields that may be recognized and interpreted by the SMTP server or mail reader. The most common example of this is the field X-Mailer: which most email clients now add to identify the name of the software or server used to generate the message. For example, here are two commonly seen X-Mailer: headers:

X-Mailer: Microsoft Outlook Express 5.00.2314.1300

X-Mailer: QUALCOMM Windows Eudora Version 5.0

EMAILX adds its own X-Mailer header indicating its version number.