Whether or not a person can view HTML in email is in no way controlled by the mail server. That is a setting that is controlled client-side.
With that aside.
Here is the excerpt directly from the the documentation of the mail() function located on php.net (which you probably didn't read)
/ To send HTML mail, you can set the Content-type header. /
$headers = "MIME-Version: 1.0\r\n";
$headers .= "Content-type: text/html; charset=iso-8859-1\r\n";
Tony, RTFM.