theming of email body

booking.module

@@ -869,6 +869,32 @@ function booking_mail($key, &$message, $params) {
   }
 }
 
+/**
+ * Implements hook_mail_alter()
+ */
+function booking_mail_alter(&$message) {
+	// $message['id'] comes from {module_name}_{email_key}
+  if($message['id'] == 'booking_registration_mail') {
+    $headers = array(
+      'MIME-Version' => '1.0',
+      'Content-Type' => 'text/html; charset=UTF-8; format=flowed',
+      'Content-Transfer-Encoding' => '8bit',
+      'X-Mailer' => 'Drupal',
+    );
+  }else{
+    $headers = array(
+      'MIME-Version' => '1.0',
+      'Content-Type' => 'text/plain; charset=UTF-8; format=flowed; delsp=yes',
+      'Content-Transfer-Encoding' => '8bit',
+      'X-Mailer' => 'Drupal',
+    );
+  }
+ 
+  //Overwrite header message header with Content-Type accommodating for html format
+  foreach($headers as $key => $value) {
+    $message['headers'][$key] = $value;
+  }
+}
 
 /**
  * Implements hook_library().