Enable usage of relative urls by prepending referer on relative paths

src/main/java/de/nclazz/service/mailrelay/adapter/web/MessageFormController.java

@@ -47,7 +47,7 @@ public class MessageFormController {
             redirect = form.getOnSuccess();
         }
 
-        return "redirect:" + redirect;
+        return "redirect:" + addRefererToRelativeUrl(referer, redirect);
     }
 
     private String formatErrorRedirect(String referer, MessageForm form, BindingResult bindingResult) {
@@ -58,7 +58,14 @@ public class MessageFormController {
                 .collect(Collectors.joining("&"));
         String origin = form.getOnError() != null ? form.getOnError() : referer;
 
-        return origin + "?" + queryParams;
+        return addRefererToRelativeUrl(referer, origin) + "?" + queryParams;
+    }
+
+    private String addRefererToRelativeUrl(String referer, String url) {
+        if(!url.startsWith("/")) {
+            return url;
+        }
+        return referer + url;
     }
 
     private static String mapValidationErrorToQueryParam(ValidationError error) {