diff -urN ../postfixadmin-3.0.orig/functions.inc.php ./functions.inc.php
--- ../postfixadmin-3.0.orig/functions.inc.php	2016-09-10 23:02:33.000000000 +0300
+++ ./functions.inc.php	2016-09-17 11:53:26.000000000 +0300
@@ -199,7 +199,7 @@
  * 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));
     }
 
@@ -218,7 +218,11 @@
             } elseif (checkdnsrr($domain,'MX')) {
                 $retval = '';
             } else {
-                $retval = sprintf(Config::lang('pInvalidDomainDNS'), htmlentities($domain));
+                if ($domain == "localhost") {
+                  $retval = '';
+                } else {
+                  $retval = sprintf(Config::lang('pInvalidDomainDNS'), htmlentities($domain));
+                }
             }
 
             $end = microtime(true); # check for slow nameservers, part 2