html-mail, content-type

I’m using a script that sends an e-mail.
The problem is that Entourage/Outlook shows
the mail in plain text. Below are the content-
type from the script. I’ve tried with different
modifications but it doesn’t work..
Does anyone know what to write to recieve the
mail in html? Thanks!

$content_type = “Content-Type: text/html\n\n”;