I should add a little note that using this method is a sure way to get your email caught by SpamAssassin. The default SpamAssassin configuration flags HTML-only emails with almost all the points needed to trigger the "spam" level.
In my experience I stay away from HTML-formatted emails using ns_sendmail for this reason.
I haven't studied the code at all, but as far as I understand, acs-mail or acs-mail-lite should be able to handle properly formatted mime-encoded email messages.
Paul.
