It's unsupported option. See Re: Launch Default Email Thread #673868 Message #833117.

>
>Is there any prog. way to send e-mail by outlook using HTML message body:
>
>I tried in two ways..
>1.Giving to my lcMessageBody variable the HTML code
>2.by parameters using the expression "&attach=myfile"
>
>But, no one works..
>
<snip>