diff -urN ../postfixadmin-3.2.orig/functions.inc.php ./functions.inc.php
--- ../postfixadmin-3.2.orig/functions.inc.php	2018-05-02 23:44:16.000000000 +0300
+++ ./functions.inc.php	2019-06-28 14:44:52.922305000 +0300
@@ -216,7 +216,7 @@
  * TODO: skip DNS check if the domain exists in PostfixAdmin?
  */
 function check_domain($domain) {
-    if (!preg_match('/^([-0-9A-Z]+\.)+' . '([-0-9A-Z]){2,13}$/i', ($domain))) {
+    if (!preg_match('/^(([-0-9A-Z]+\.)+' . '([-0-9A-Z]){2,13}|localhost)$/i', ($domain))) {
         return sprintf(Config::lang('pInvalidDomainRegex'), htmlentities($domain));
     }