This form enables you to send mail via an XML-RPC server. For public use only the "Userland" server will work (see Dave Winer's message). When you press Send this page will reload, showing you the XML-RPC request sent to the host server, the XML-RPC response received and the internal evaluation done by the PHP implementation.
You can find the source to this page here: mail.php
And the source to a functionally identical mail-by-XML-RPC server in the file server.php included with the library (look for the 'mail_send' method)