For any email HTML formatted message in Outlook Express open Properties - Details - Message Source.
It gives you an understanding how such message should be formatted.

You will see something like

Content-Type: multipart/alternative;
  boundary="NextPart_048F8BC8A2197DE2036A"

This is multi-part message in MIME format.

--NextPart_048F8BC8A2197DE2036A
Content-Type: text/plain; chars

and then

--NextPart_048F8BC8A2197DE2036A
Content-Type: text/html; charset=iso-8859-1

Content-Type value is important.

For MAPI and Winsock you have to create all parts by yourself. CDO approach is more convenient:

#DEFINE cret  Chr(13)+Chr(10)
LOCAL cdoMsg
cdoMsg = CreateObject('CDO.Message')

WITH cdoMsg
	.Sender = 'info@myhost.com'    && replace with your address
	.To = 'myfriend@somewhere.com' && replace with valid recipient email

	.Subject = 'Message in HTML format sent using Microsoft CDO'

	.HTMLBody = '<h1>What could be simpler?</h1>' +;
		'<p>Sending emails with <Strong>Microsoft CDO</Strong> appears to be ' +;
		'the <Em>simplest</Em> way you have ever tested.</p>'

	WAIT WINDOW NOWAIT 'Sending message...'
	.Send
	WAIT CLEAR
ENDWITH

Just using HTMLBody property instead of TextBody creates HTML formatted email message automatically.