diff --git a/data/conf/dovecot/auth/mailcowauth.php b/data/conf/dovecot/auth/mailcowauth.php
index d1c7381f6..c8a73e39e 100644
--- a/data/conf/dovecot/auth/mailcowauth.php
+++ b/data/conf/dovecot/auth/mailcowauth.php
@@ -101,7 +101,16 @@ if ($result === false){
   // Init Identity Provider
   $iam_provider = identity_provider('init');
   $iam_settings = identity_provider('get');
-  $result = user_login($post['username'], $post['password'], array('is_internal' => true, 'service' => $post['service']));
+  try {
+    $result = user_login($post['username'], $post['password'], array('is_internal' => true, 'service' => $post['service']));
+  } catch (\Mailcow\Auth\IdpUnavailable $e) {
+    // temporary failure for Dovecot, no failed login
+    error_log("MAILCOWAUTH: Identity provider unavailable for user " . $post['username'] . " with service " . $post['service'] . " from IP " . $post['real_rip'] . ": " . $e->getMessage());
+    http_response_code(503); // Service Unavailable
+    echo json_encode($return);
+    session_destroy();
+    exit;
+  }
   if ($result) {
     error_log('MAILCOWAUTH: User auth for user ' . $post['username'] . " with service " . $post['service'] . " from IP " . $post['real_rip']);
     set_sasl_log($post['username'], $post['real_rip'], $post['service']);
diff --git a/data/conf/dovecot/auth/passwd-verify.lua b/data/conf/dovecot/auth/passwd-verify.lua
index a22488ea8..ddd5a316a 100644
--- a/data/conf/dovecot/auth/passwd-verify.lua
+++ b/data/conf/dovecot/auth/passwd-verify.lua
@@ -41,6 +41,12 @@ function auth_password_verify(request, password)
     return dovecot.auth.PASSDB_RESULT_INTERNAL_FAILURE, "Upstream unreachable"
   end
 
+  -- 5xx: the backend or the identity provider cannot answer, a temporary failure and no failed login
+  if c >= 500 then
+    dovecot.i_info("HTTP request to auth backend failed with " .. tostring(c) .. " for user " .. request.user)
+    return dovecot.auth.PASSDB_RESULT_INTERNAL_FAILURE, "Upstream unavailable"
+  end
+
   if c ~= 200 and c ~= 401 then
     dovecot.i_info("HTTP request failed with " .. tostring(c) .. " for user " .. request.user)
     return dovecot.auth.PASSDB_RESULT_PASSWORD_MISMATCH, "Upstream error"
diff --git a/data/conf/dovecot/dovecot.conf b/data/conf/dovecot/dovecot.conf
index 05d9264fa..c3789fb24 100644
--- a/data/conf/dovecot/dovecot.conf
+++ b/data/conf/dovecot/dovecot.conf
@@ -67,9 +67,11 @@ passdb {
 }
 # check for regular password - if empty (e.g. force-passwd-reset), previous pass=yes passdbs also fail
 # a return of the following passdb is mandatory
+# cache_key with password: the lua default (%u) would let a temporary failure fall back to the negative entry
+# of a passdb lookup (master logins) and log "unknown user", which netfilter counts as failed login
 passdb {
   driver = lua
-  args = file=/etc/dovecot/auth/passwd-verify.lua blocking=yes
+  args = file=/etc/dovecot/auth/passwd-verify.lua blocking=yes cache_key=%s:%u:%w
 }
 # Set doveadm_password=your-secret-password in data/conf/dovecot/extra.conf (create if missing)
 service doveadm {
diff --git a/data/web/inc/functions.auth.inc.php b/data/web/inc/functions.auth.inc.php
index a507bbfa4..87566af54 100644
--- a/data/web/inc/functions.auth.inc.php
+++ b/data/web/inc/functions.auth.inc.php
@@ -36,7 +36,18 @@ function check_login($user, $pass, $extra = null) {
 
   // Try validate user
   if (!isset($role) || $role == "user") {
-    $result = user_login($user, $pass, $extra);
+    try {
+      $result = user_login($user, $pass, $extra);
+    } catch (\Mailcow\Auth\IdpUnavailable $e) {
+      // no failed login, netfilter must not count it
+      error_log("mailcow UI: Identity provider unavailable for " . $user . " by " . $_SERVER['REMOTE_ADDR'] . ": " . $e->getMessage());
+      $_SESSION['return'][] =  array(
+        'type' => 'danger',
+        'log' => array(__FUNCTION__, $user, '*', $e->getMessage()),
+        'msg' => 'iam_unavailable'
+      );
+      return false;
+    }
     if ($result !== false) {
       $real_rip = ($_SERVER['HTTP_X_REAL_IP'] ?? $_SERVER['REMOTE_ADDR']);
       set_sasl_log($user, $real_rip, $extra['service']);
@@ -516,11 +527,14 @@ function keycloak_mbox_login_rest($user, $pass, $extra = null){
     return false;
   }
   if (!$iam_provider) {
-    return false;
+    throw new \Mailcow\Auth\IdpUnavailable('Keycloak is not configured completely');
   }
 
   // get access_token for service account of mailcow client
   $admin_token = identity_provider("get-keycloak-admin-token");
+  if (!$admin_token) {
+    throw new \Mailcow\Auth\IdpUnavailable('Could not get an admin token from Keycloak');
+  }
 
   // get the mailcow_password attribute from keycloak user
   $url = "{$iam_settings['server_url']}/admin/realms/{$iam_settings['realm']}/users";
@@ -541,13 +555,9 @@ function keycloak_mbox_login_rest($user, $pass, $extra = null){
   $users = json_decode(curl_exec($curl), true);
   $code = curl_getinfo($curl, CURLINFO_HTTP_CODE);
   curl_close($curl);
+  // the password is checked below, a failed request is never a wrong password
   if ($code != 200) {
-    $_SESSION['return'][] =  array(
-      'type' => 'danger',
-      'log' => array(__FUNCTION__, $user, '*', 'Identity Provider returned HTTP ' . $code),
-      'msg' => 'generic_server_error'
-    );
-    return false;
+    throw new \Mailcow\Auth\IdpUnavailable('Keycloak returned HTTP ' . $code);
   }
   // exact=true can still return several users with that email
   if (!is_array($users) || count($users) != 1) {
@@ -687,7 +697,8 @@ function ldap_mbox_login($user, $pass, $extra = null){
     return false;
   }
   if (!$iam_provider) {
-    return false;
+    // identity_provider('init') could not connect
+    throw new \Mailcow\Auth\IdpUnavailable('No connection to the LDAP server');
   }
 
   try {
@@ -700,33 +711,17 @@ function ldap_mbox_login($user, $pass, $extra = null){
         \Mailcow\Identity\Directory\LdapDirectory::idAttributes($iam_settings['id_attribute'] ?? '')))));
 
     $user_res = $ldap_query->firstOrFail();
-  } catch (Exception $e) {
-    // clear $_SESSION['return'] to not leak data
-    $_SESSION['return'] = array();
-    $_SESSION['return'][] =  array(
-      'type' => 'danger',
-      'log' => array(__FUNCTION__, $user, '*', $e->getMessage()),
-      'msg' => 'generic_server_error'
-    );
+  } catch (\LdapRecord\Query\ObjectNotFoundException $e) {
     return false;
-  }
-  try {
-    if (!$iam_provider->auth()->attempt($user_res['dn'], $pass)) {
-      return false;
-    }
   } catch (Exception $e) {
-    // clear $_SESSION['return'] to not leak data
-    $_SESSION['return'] = array();
-    $_SESSION['return'][] =  array(
-      'type' => 'danger',
-      'log' => array(__FUNCTION__, $user, '*', $e->getMessage()),
-      'msg' => 'generic_server_error'
-    );
+    throw new \Mailcow\Auth\IdpUnavailable('LDAP search failed: ' . $e->getMessage(), 0, $e);
+  }
+  $idp_login = \Mailcow\Auth\AuthModule::fromGlobals()->idp;
+  if (!$idp_login->verifyLdapPassword($iam_provider, (string)$user_res['dn'], (string)$pass)) {
     return false;
   }
 
   // get mapped template
-  $idp_login = \Mailcow\Auth\AuthModule::fromGlobals()->idp;
   $mapped_template = $idp_login->mappedTemplate($user_res[$iam_settings['attribute_field']] ?? array());
   $external_id = \Mailcow\Identity\Directory\LdapDirectory::entryId($user_res, $iam_settings['id_attribute'] ?? '');
 
diff --git a/data/web/inc/functions.inc.php b/data/web/inc/functions.inc.php
index 660cd67f0..79bc97b24 100644
--- a/data/web/inc/functions.inc.php
+++ b/data/web/inc/functions.inc.php
@@ -1292,8 +1292,13 @@ function set_tfa($_data) {
       $row = $stmt->fetch(PDO::FETCH_ASSOC);
       if ($row) {
         if ($row['authsource'] == 'ldap'){
-          if (!ldap_mbox_login($username, $_data["confirm_password"], $iam_settings)) $access_denied = true;
-          else $access_denied = false;
+          try {
+            if (!ldap_mbox_login($username, $_data["confirm_password"], $iam_settings)) $access_denied = true;
+            else $access_denied = false;
+          } catch (\Mailcow\Auth\IdpUnavailable $e) {
+            $_SESSION['return'][] = array('type' => 'danger', 'log' => array(__FUNCTION__, $_data_log, $e->getMessage()), 'msg' => 'iam_unavailable');
+            return false;
+          }
         } else {
           if (!verify_hash($row['password'], $_data["confirm_password"])) $access_denied = true;
           else $access_denied = false;
diff --git a/data/web/inc/lib/mailcow/Auth/IdpUnavailable.php b/data/web/inc/lib/mailcow/Auth/IdpUnavailable.php
new file mode 100644
index 000000000..6dae16b48
--- /dev/null
+++ b/data/web/inc/lib/mailcow/Auth/IdpUnavailable.php
@@ -0,0 +1,15 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Mailcow\Auth;
+
+use RuntimeException;
+
+/**
+ * The identity provider could not answer a login (unreachable, timeout, server error). No failed login:
+ * callers report a temporary failure and must not count it as an attempt for netfilter.
+ */
+class IdpUnavailable extends RuntimeException
+{
+}
diff --git a/data/web/inc/lib/mailcow/Auth/Service/IdpLoginService.php b/data/web/inc/lib/mailcow/Auth/Service/IdpLoginService.php
index bc9833398..2deb7ef27 100644
--- a/data/web/inc/lib/mailcow/Auth/Service/IdpLoginService.php
+++ b/data/web/inc/lib/mailcow/Auth/Service/IdpLoginService.php
@@ -4,6 +4,10 @@ declare(strict_types=1);
 
 namespace Mailcow\Auth\Service;
 
+use LdapRecord\Auth\BindException;
+use LdapRecord\Connection;
+use LdapRecord\LdapRecordException;
+use Mailcow\Auth\IdpUnavailable;
 use Mailcow\Identity\Model\Settings;
 use Mailcow\Identity\Service\LinkService;
 use Mailcow\MailboxGroup\Service\TemplateService;
@@ -19,6 +23,37 @@ class IdpLoginService
     {
     }
 
+    /**
+     * Binds as the user to check the password, then again as the configured user. False for wrong credentials,
+     * IdpUnavailable when the server cannot answer (client side codes are negative: server down, timeout, ...).
+     */
+    public function verifyLdapPassword(Connection $ldap, string $dn, string $password): bool
+    {
+        // an empty password would be an unauthenticated bind, which succeeds
+        if ($dn === '' || $password === '') {
+            return false;
+        }
+        $guard = $ldap->auth();
+        try {
+            $guard->bind($dn, $password);
+            $bound = true;
+        } catch (BindException $e) {
+            $code = $e->getDetailedError()?->getErrorCode();
+            if ($code !== null && ($code < 0 || in_array($code, [51, 52], true))) {
+                throw new IdpUnavailable('LDAP bind failed: ' . $e->getMessage(), 0, $e);
+            }
+            $bound = false;
+        } catch (LdapRecordException $e) {
+            throw new IdpUnavailable('LDAP bind failed: ' . $e->getMessage(), 0, $e);
+        }
+        try {
+            $guard->bindAsConfiguredUser();
+        } catch (LdapRecordException $e) {
+            // the login needs no further LDAP access
+        }
+        return $bound;
+    }
+
     /** Template of the first attribute mapping matching one of the IdP attribute values, null if none */
     public function mappedTemplate(mixed $values): ?string
     {