diff -urN ../postfixadmin-cd73f275.orig/functions.inc.php ./functions.inc.php
--- ../postfixadmin-cd73f275.orig/functions.inc.php	2021-09-02 23:45:38.000000000 +0300
+++ ./functions.inc.php	2022-02-03 21:50:17.273507000 +0200
@@ -242,12 +242,16 @@
  */
 function check_domain($domain)
 {
-    if (!preg_match('/^([-0-9A-Z]+\.)+' . '([-0-9A-Z]){1,13}$/i', ($domain))) {
+    if (!preg_match('/^(([-0-9A-Z]+\.)+' . '([-0-9A-Z]){1,13}|localhost)$/i', ($domain))) {
         return sprintf(Config::lang('pInvalidDomainRegex'), htmlentities($domain));
     }
 
     if (Config::bool('emailcheck_resolve_domain') && 'WINDOWS'!=(strtoupper(substr(php_uname('s'), 0, 7)))) {
 
+        if ($domain == 'localhost') {
+            return '';
+        }
+
         // Look for an AAAA, A, or MX record for the domain
 
         if (function_exists('checkdnsrr')) {
@@ -323,7 +327,7 @@
         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);