diff -urN ../postfixadmin-3.2.4.orig/functions.inc.php ./functions.inc.php
--- ../postfixadmin-3.2.4.orig/functions.inc.php	2020-04-17 11:15:04.000000000 +0300
+++ ./functions.inc.php	2020-09-01 00:18:23.564345000 +0300
@@ -216,10 +216,11 @@
  * 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));
     }
 
+    if ($domain != 'localhost')
     if (Config::bool('emailcheck_resolve_domain') && 'WINDOWS'!=(strtoupper(substr(php_uname('s'), 0, 7)))) {
 
         // Look for an AAAA, A, or MX record for the domain
@@ -277,7 +278,7 @@
         return Config::lang_f('pInvalidMailRegex', $email);
     }
 
-    if (function_exists('filter_var')) {
+    if ((function_exists('filter_var')) and (!preg_match('/^\S+@localhost$/', $email))) {
         $check = filter_var($email, FILTER_VALIDATE_EMAIL);
         if (!$check) {
             return Config::lang_f('pInvalidMailRegex', $email);