MAPI control uses Simple MAPI, which does not handle HTML. There is a trick when using Simple MAPI directly (MAPISendMail) - set the body to NULL and attach and HTML file: it will be used as the message body. I don't know if that trick will work with the MAPI control.
Why not switch to using the Outlook Object Model? It is perfectly capable of handling HTML:
set App = CreateObject("Outlook.Application")
set NS = App.GetNmaespace("MAPI")
NS.Logon
set Msg = App.CreateItem(0)
Msg.To = sSendTo
Msg.Subject = sSubject
Msg.HTMLBody = sYourHTMLBody
Msg.Send 'or Msg.Display