Set the return path via the envelope sender

assets/php/contact_form.php

@@ -34,17 +34,25 @@ function prepare_message_body(string $message, string $name) {
  *  phpmailer: Bibliothek, der per Composer installiert wird. Tut ganz gut mit SMTP.
  */
 function send_message_to_office(string $subject, string $message, string $name, string $email) {
+    $returnPath = filter_var(getenv('WTF_RETURN_PATH'), FILTER_VALIDATE_EMAIL);
+    $to = filter_var(getenv('WTF_CONTACT_TO'), FILTER_VALIDATE_EMAIL);
+
+    if (!$returnPath || !$to) {
+        error_log('Address for "To" or "Return-Path" is invalid');
+        return false;
+    }
+
     return mail(
-        getenv('WTF_CONTACT_TO'),
+        $to,
         "=?UTF-8?B?" . base64_encode($subject) . "?=",
         prepare_message_body($message, $name),
-        $additional_headers = array(
+        array(
             "From" => getenv('WTF_CONTACT_FROM'),
             "Reply-To" => $email,
-            "Return-Path" => getenv('WTF_RETURN_PATH'),
             "Content-Type" => "text/plain; charset=utf-8",
             "Content-Transfer-Encoding" => "base64",
         ),
+        "-f $returnPath"
     );
 }