diff -urN ../postfixadmin-2.93.orig/functions.inc.php ./functions.inc.php
--- ../postfixadmin-2.93.orig/functions.inc.php	2015-10-30 19:36:29.000000000 +0200
+++ ./functions.inc.php	2015-10-30 20:08:46.000000000 +0200
@@ -198,7 +198,7 @@
  * 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));
     }