@Daniel
For OpenURL to work you might need to do some configuration. Launch the pref program go to the browser tab, if OWB entry exists check its checkbox (if not already, and uncheck all the other) and verify path is correct (correct it if not), then save.
Launch a shell type
C:OpenURL http://www.amigans.net
and see what happens, if OWB launches and display the page then OpenURL.library is correctly configured, your problems comes from SimpleMail, if not keep modifying OpenURL preferences until it works, then try SimpleMail.