diff -urN ../roundcubemail-1.6.2.orig/program/lib/Roundcube/rcube_ldap.php ./program/lib/Roundcube/rcube_ldap.php
--- ../roundcubemail-1.6.2.orig/program/lib/Roundcube/rcube_ldap.php	2023-07-01 14:29:36.000000000 +0300
+++ ./program/lib/Roundcube/rcube_ldap.php	2023-07-03 22:15:06.391155000 +0300
@@ -307,6 +307,19 @@
             $this->prop['hosts'] = [];
         }
 
+        $this->ldap->_debug("Check LDAP hosts list against broken hosts list.");
+        if (!isset($_SESSION['roundcube_ldap_hosts_probably_broken']) or !is_array($_SESSION['roundcube_ldap_hosts_probably_broken']) or (count($_SESSION['roundcube_ldap_hosts_probably_broken']) == 0)) {
+            $this->ldap->_debug("Broken LDAP hosts list is empty.");
+        } else {
+            $hosts_checked = array_diff($this->prop['hosts'], $_SESSION['roundcube_ldap_hosts_probably_broken']);
+            if (count($hosts_checked) == 0) {
+                $this->ldap->_debug("All hosts are broken. Reset broken hosts list.");
+                $_SESSION['roundcube_ldap_hosts_probably_broken'] = [];
+            } else {
+                $this->prop['hosts'] = $hosts_checked;
+            }
+        }
+
         // try to connect + bind for every host configured
         // with OpenLDAP 2.x ldap_connect() always succeeds but ldap_bind will fail if host isn't reachable
         // see http://www.php.net/manual/en/function.ldap-connect.php
diff -urN ../roundcubemail-1.6.2.orig/vendor/kolab/net_ldap3/lib/Net/LDAP3.php ./vendor/kolab/net_ldap3/lib/Net/LDAP3.php
--- ../roundcubemail-1.6.2.orig/vendor/kolab/net_ldap3/lib/Net/LDAP3.php	2023-07-01 14:29:39.000000000 +0300
+++ ./vendor/kolab/net_ldap3/lib/Net/LDAP3.php	2023-07-03 22:16:40.024998000 +0300
@@ -66,6 +66,8 @@
         'v' => 'read'
     ];
 
+    private $hosts_used = [];
+
     /*
      * Manipulate configuration through the config_set and config_get methods.
      * Available options:
@@ -518,6 +520,18 @@
         $this->_debug("S: ".ldap_error($this->conn));
         $this->_error("LDAP: Bind failed for dn=$bind_dn. ".ldap_error($this->conn));
 
+        if ((ldap_errno($this->conn) == -1) or (ldap_error($this->conn) == "Can't contact LDAP server")) {
+            if (isset($_SESSION['roundcube_ldap_hosts_probably_broken']) and is_array($_SESSION['roundcube_ldap_hosts_probably_broken'])) {
+                if (!in_array($this->_current_host, $_SESSION['roundcube_ldap_hosts_probably_broken'])) {
+                    $this->_debug(sprintf("Save host %s as broken", $this->_current_host));
+                    $_SESSION['roundcube_ldap_hosts_probably_broken'][] = $this->_current_host;
+                }
+            } else {
+                $this->_debug(sprintf("Save host %s as broken", $this->_current_host));
+                $_SESSION['roundcube_ldap_hosts_probably_broken'] = [$this->_current_host];
+            }
+        }
+
         return false;
     }
 
@@ -605,11 +619,23 @@
         }
 
         if (isset($this->conn) && $this->conn !== false) {
-            $this->_debug("Connection already exists");
-            return true;
+            if ($this->_current_bind_dn) {
+                $this->_debug("Connection already exists");
+                return true;
+            } else {
+                $hosts = (!empty($host) ? (is_array($host) ? $host : [$host]) : $this->config_get('hosts', []));
+                $hosts = array_diff($hosts, $this->hosts_used);
+                if (count($hosts) == 0) {
+                    $this->_debug("All LDAP hosts are broken");
+                    return true;
+                } else {
+                    $this->_debug(sprintf("C: Try next LDAP host %s", $hosts[0]));
+                }
+            }
+        } else {
+            $hosts = (!empty($host) ? $host : $this->config_get('hosts', []));
         }
 
-        $hosts = !empty($host) ? $host : $this->config_get('hosts', []);
         $port  = $this->config_get('port', 389);
 
         foreach ((array) $hosts as $host) {
@@ -617,6 +643,7 @@
             $this->_debug("C: Connect [{$ldap_uri}]");
 
             if ($lc = @ldap_connect($ldap_uri)) {
+                $this->hosts_used[] = $host;
                 if ($this->config_get('use_tls', false) === true) {
                     if (!ldap_start_tls($lc)) {
                         $this->_debug("S: Could not start TLS. " . ldap_error($lc));
@@ -2616,7 +2643,7 @@
         $this->__log(LOG_CRIT, func_get_args());
     }
 
-    protected function _debug()
+    public function _debug()
     {
         $this->__log(LOG_DEBUG, func_get_args());
     }