New_user_approve_email_header
Modify The List Of Headers That Are Sent As Part Of The Email Messages
/**
* Add a new header
* $headers an array of the current email headers
*/
function add_email_header( $header ) {
$header[] = "Content-Type: text/html; charset=\"" . get_option( 'blog_charset' ) . "\"\n";
}
add_filter( 'new_user_approve_email_header', 'add_email_header' );