diff --git a/program/actions/mail/mark.php b/program/actions/mail/mark.php
index 0355b19ea..4df3b18a4 100644
--- a/program/actions/mail/mark.php
+++ b/program/actions/mail/mark.php
@@ -1,199 +1,200 @@
 <?php
 
 /**
  +-----------------------------------------------------------------------+
  | This file is part of the Roundcube Webmail client                     |
  |                                                                       |
  | Copyright (C) The Roundcube Dev Team                                  |
  |                                                                       |
  | Licensed under the GNU General Public License version 3 or            |
  | any later version with exceptions for skins & plugins.                |
  | See the README file for a full license statement.                     |
  |                                                                       |
  | PURPOSE:                                                              |
  |   Mark the submitted messages with the specified flag                 |
  +-----------------------------------------------------------------------+
  | Author: Thomas Bruederli <roundcube@gmail.com>                        |
  +-----------------------------------------------------------------------+
 */
 
 class rcmail_action_mail_mark extends rcmail_action_mail_index
 {
     protected static $mode = self::MODE_AJAX;
 
     /**
      * Request handler.
      *
      * @param array $args Arguments from the previous step(s)
      */
     public function run($args = [])
     {
         $rcmail  = rcmail::get_instance();
         $_uids   = rcube_utils::get_input_value('_uid', rcube_utils::INPUT_POST);
         $flag    = rcube_utils::get_input_value('_flag', rcube_utils::INPUT_POST);
         $folders = rcube_utils::get_input_value('_folders', rcube_utils::INPUT_POST);
         $mbox    = rcube_utils::get_input_value('_mbox', rcube_utils::INPUT_POST);
 
         if (empty($_uids) || empty($flag)) {
             $rcmail->output->show_message('internalerror', 'error');
             $rcmail->output->send();
         }
 
         $rcmail       = rcmail::get_instance();
         $threading    = (bool) $rcmail->storage->get_threading();
         $skip_deleted = (bool) $rcmail->config->get('skip_deleted');
         $read_deleted = (bool) $rcmail->config->get('read_when_deleted');
 
         $flags_map  = [
             'undelete'  => 'UNDELETED',
             'delete'    => 'DELETED',
             'read'      => 'SEEN',
             'unread'    => 'UNSEEN',
             'flagged'   => 'FLAGGED',
             'unflagged' => 'UNFLAGGED',
         ];
 
         $flag      = self::imap_flag($flag);
         $old_count = 0;
+        $post_from = isset($_POST['_from']) ? $_POST['_from'] : null;
 
-        if ($flag == 'DELETED' && $skip_deleted && $_POST['_from'] != 'show') {
+        if ($flag == 'DELETED' && $skip_deleted && $post_from != 'show') {
             // count messages before changing anything
             $old_count = $rcmail->storage->count(null, $threading ? 'THREADS' : 'ALL');
             $old_pages = ceil($old_count / $rcmail->storage->get_pagesize());
         }
 
         if ($folders == 'all') {
             $mboxes = $rcmail->storage->list_folders_subscribed('', '*', 'mail');
             $input  = array_combine($mboxes, array_fill(0, count($mboxes), '*'));
         }
         else if ($folders == 'sub') {
             $delim  = $rcmail->storage->get_hierarchy_delimiter();
             $mboxes = $rcmail->storage->list_folders_subscribed($mbox . $delim, '*', 'mail');
             array_unshift($mboxes, $mbox);
             $input = array_combine($mboxes, array_fill(0, count($mboxes), '*'));
         }
         else if ($folders == 'cur') {
             $input = [$mbox => '*'];
         }
         else {
             $input = self::get_uids(null, null, $dummy, rcube_utils::INPUT_POST);
         }
 
         $marked = 0;
         $count  = 0;
         $read   = 0;
 
         foreach ($input as $mbox => $uids) {
             $marked += (int) $rcmail->storage->set_flag($uids, $flag, $mbox);
             $count  += is_array($uids) ? count($uids) : 1;
         }
 
         if (!$marked) {
             // send error message
-            if (empty($_POST['_from']) || $_POST['_from'] != 'show') {
+            if ($post_from != 'show') {
                 $rcmail->output->command('list_mailbox');
             }
 
             self::display_server_error('errormarking');
             $rcmail->output->send();
         }
         else if (empty($_POST['_quiet'])) {
             $rcmail->output->show_message('messagemarked', 'confirmation');
         }
 
         if ($flag == 'DELETED' && $read_deleted && !empty($_POST['_ruid'])) {
             if ($ruids = rcube_utils::get_input_value('_ruid', rcube_utils::INPUT_POST)) {
                 foreach (self::get_uids($ruids) as $mbox => $uids) {
                     $read += (int) $rcmail->storage->set_flag($uids, 'SEEN', $mbox);
                 }
             }
 
             if ($read && !$skip_deleted) {
                 $rcmail->output->command('flag_deleted_as_read', $ruids);
             }
         }
 
         if ($flag == 'SEEN' || $flag == 'UNSEEN' || ($flag == 'DELETED' && !$skip_deleted)) {
             foreach ($input as $mbox => $uids) {
                 self::send_unread_count($mbox);
             }
 
             $rcmail->output->set_env('last_flag', $flag);
         }
         else if ($flag == 'DELETED' && $skip_deleted) {
-            if ($_POST['_from'] == 'show') {
+            if ($post_from == 'show') {
                 if ($next = rcube_utils::get_input_value('_next_uid', rcube_utils::INPUT_GPC)) {
                     $rcmail->output->command('show_message', $next);
                 }
                 else {
                     $rcmail->output->command('command', 'list');
                 }
             }
             else {
                 $search_request = rcube_utils::get_input_value('_search', rcube_utils::INPUT_GPC);
 
                 // refresh saved search set after moving some messages
                 if ($search_request && $rcmail->storage->get_search_set()) {
                     $_SESSION['search'] = $rcmail->storage->refresh_search();
                 }
 
                 $msg_count      = $rcmail->storage->count(NULL, $threading ? 'THREADS' : 'ALL');
                 $page_size      = $rcmail->storage->get_pagesize();
                 $page           = $rcmail->storage->get_page();
                 $pages          = ceil($msg_count / $page_size);
                 $nextpage_count = $old_count - $page_size * $page;
                 $remaining      = $msg_count - $page_size * ($page - 1);
                 $jump_back      = false;
 
                 // jump back one page (user removed the whole last page)
                 if ($page > 1 && $remaining == 0) {
                     $page -= 1;
                     $rcmail->storage->set_page($page);
                     $_SESSION['page'] = $page;
                     $jump_back = true;
                 }
 
                 foreach ($input as $mbox => $uids) {
                     self::send_unread_count($mbox, true);
                 }
 
                 // update message count display
                 $rcmail->output->set_env('messagecount', $msg_count);
                 $rcmail->output->set_env('current_page', $page);
                 $rcmail->output->set_env('pagecount', $pages);
                 $rcmail->output->command('set_rowcount', self::get_messagecount_text($msg_count), $mbox);
 
                 if ($threading) {
                     $count = rcube_utils::get_input_value('_count', rcube_utils::INPUT_POST);
                 }
 
                 // add new rows from next page (if any)
                 if ($old_count && $_uids != '*' && ($jump_back || $nextpage_count > 0)) {
                     // #5862: Don't add more rows than it was on the next page
                     $count = $jump_back ? null : min($nextpage_count, $count);
 
                     $a_headers = $rcmail->storage->list_messages($mbox, null,
                         self::sort_column(), self::sort_order(), $count);
 
                     self::js_message_list($a_headers, false);
                }
             }
         }
 
         $rcmail->output->send();
     }
 
     public static function imap_flag($flag)
     {
         $flags_map  = [
             'undelete'  => 'UNDELETED',
             'delete'    => 'DELETED',
             'read'      => 'SEEN',
             'unread'    => 'UNSEEN',
             'flagged'   => 'FLAGGED',
             'unflagged' => 'UNFLAGGED',
         ];
 
         return !empty($flags_map[$flag]) ? $flags_map[$flag] : strtoupper($flag);
     }
 }