XBYJS is a command that instructs the www4mail server
to request that your web browser to attempt to mail
using javascript.

Example
XBYJS
http://www.ictp.trieste.it

NOTE: This command is useful for those web browsers that
normally cannot place form data directly into e-mail body.
This option would still generate a blank body but the subject
line should be filled in.
You might have to enable support for Javascript, please verify the
options of the IE web browser.