At the moment URLOpen simply passes your command line to
the client as is.
The individual client must accept the keywords you use
as a command template and the commands must be formatted
the same way you would use it as a shell command that
calls ReadArgs() to get the arguments parsed.
If the client does not respond when invoking it from a
shell, then urlopen can't use it.
Try typing; Appdir:SimpleMail ?
to see what option it supports, I don't have it here to
see, however, I did a quick test with YAM with the
following line works;
urlopen
mailto=info@example.com subject="The subject" attach=temp:domains.html
PS:
YAM doesn't appear to have a "Body" keyword, but it does
have a "Letter" keyword, however it doesn't appear to work
on the version of YAM I have here for some reason.