"X-Wss-Id:" ; Worldtalk gateways
"X-X-Sender:" ; http://people.dsv.su.se/~jpalme/ietf/mail-headers/
"X-XPT-XSL-Name:" ; Paypal http://www.paypal.com
+ "X-xsi-"
"X-XWALL-" ; http://www.dataenter.co.at/doc/xwall_undocumented_config.htm
"X-Y-GMX-Trusted:" ; http://www.gmx.net/
"X-Yahoo"