As far as I know, there isn't a way to do this, at least on our side. There may be a way to tell Outlook to convert email sent via MAPI into HTML, but I'm not an expert there so I don't know for sure.
(By the way, the "/O" should not be part of the command. Just Copy SENDMAIL on its own is the correct form, or Copy SENDMAIL="joe.blogs@example.com" to specify a recipient via the command.)