|Some information is available if you are prepared to
read the RFC's.
For those less ambitious here's a link to a site that has less technical information.
Note: The "X" headers are defined by the email application and so are not standardized therefore there
cannot be a comprehensive list of them. In the case of Poco Slaven couldn't even produce such a list as
any script writer could add an "X" header at any time, in fact users can define their own headers via the
Optional tab of the Compose window.