Sending HTML Emails

I am trying to send an email as HTML but it is coming out as plain text. I have this in my module

function redeem_mail($key, &$message, $params){
	if ($key == "voucher") {
	  $message['headers']['Content-Type'] = 'text/html; charset=UTF-8;';
	  $message['subject'] = $params['subject'];
	  $message['body'][] = $params['body'];
	}
}	

function redeem_mail_alter(&$message) {
	
	if (($message['module'] == 'redeem') && ($message['key'] = 'voucher')) {
		$message['headers'] = array(
		  'MIME-Version' => '1.0',
		  'Content-Type' => 'text/html; charset=UTF-8; format=flowed',
		  'Content-Transfer-Encoding' => '8Bit',
		  'X-Mailer' => 'Drupal',
		  'Return-Path' => $message['from'],
		);
	}
}

I don’t think it is going into redeem_mail_alter by the look of it from the tracing I tried but it looks fine to me. Any ideas please – thanks

I am calling it using

$mail_result = drupal_mail(‘redeem’, ‘voucher’, $email_to, language_default(), $params, $email_from, TRUE);

Drupal version: 


Source: https://www.drupal.org/taxonomy/term/4/feed