Merge branch 'develop' into 'develop'

Fix bug with Array-String in postfix main.cf

Closes #5872

See merge request ispconfig/ispconfig3!1309

server/plugins-available/postfix_server_plugin.inc.php

@@ -193,7 +193,7 @@ class postfix_server_plugin {
 					}
 				}
 				if ($i == count($new_options)) {
-					$new_options[] = array('reject_unknown_client_hostname');
+					$new_options[] = 'reject_unknown_client_hostname';
 				}
 
 				$app->system->exec_safe("postconf -e ?", 'smtpd_client_restrictions = '.implode(", ", $new_options));
@@ -231,7 +231,7 @@ class postfix_server_plugin {
 					}
 				}
 				if ($i == count($new_options)) {
-					$new_options[] = array('reject_unknown_helo_hostname');
+					$new_options[] = 'reject_unknown_helo_hostname';
 				}
 
 				$app->system->exec_safe("postconf -e ?", 'smtpd_helo_restrictions = '.implode(", ", $new_options));