use this header for the mail:
$header = "MIME-Version: 1.0\r\n";
$header .= "Content-type: text/html; charset: utf8\r\n";
and for the content/body:
<html>
<head>
<meta http-equiv="content-type" content="text/html; charset=utf-8" />
... ... ...
it’s important to use inline css commands and recommanded to use tables for the interface.
…
In your Mail-Body you than have to put HTML code with head and body