Mail will not send in HTML

Okay, I’ve got the following issue. I’m trying to send a mail in HTML format. I made a class that returns an HTML string, and that works great.

When I pass that html-mail as $message in my function, works also. But it will not send as html, but plain text.

Now I’ve tried the following things:

1)
$headers = array('Content-Type: text/html; charset=UTF-8');
wp_mail($to, $subject, $message, $headers);

2)
$headers="MIME-Version: 1.0" . "\r\n";
$headers .= 'Content-type: text/html; charset=iso-8859-1' . "\r\n";

3)
$headers="MIME-Version: 1.0" . "\r\n";
$headers .= 'Content-type: text/html; charset=UTF-8' . "\r\n";

4)
function wpse27856_set_content_type(){
return "text/html";
}
add_filter( 'wp_mail_content_type','wpse27856_set_content_type' );

What else could it be?

Try this one.

  add_filter( 'wp_mail_content_type', 'wpdocs_set_html_mail_content_type' );

    $to = 'sendto@example.com';
    $subject="The subject";
    $body = 'The email body content';
    $headers = array('Content-Type: text/html; charset=UTF-8');

    wp_mail( $to, $subject, $body , $headers);

    // Reset content-type to avoid conflicts -- https://core.trac.wordpress.org/ticket/23578
    remove_filter( 'wp_mail_content_type', 'wpdocs_set_html_mail_content_type' );

    function wpdocs_set_html_mail_content_type() {
        return 'text/html';
    }