The best thing you can do is look at the raw output of a HTML formatted eMail then emulate it.

Here's the basics of what I did
create the script web-email.sh with the content below
#!/usr/bin/sh
(
cat $1 | /usr/sbin/sendmail -t
)### end of script

Some caution should be observed because outlook has some weird behavior.

Keep in mind that you still need to define the sendmail To:, From:, Subject, etc which can not be in HTML format.

Example: aa.out has the contents below:

To: sombody@someplace.com
From: sombodyelse@someplace.com
Subject: HTML Email
Content-Transfer-Encoding: quoted-printable


--__________MIMEboundary__________
Content-Type: text/html; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

< the HTML YOU WANT to send in the BODY of the email goes here >

--__________MIMEboundary__________--