quietone
--- a/core/modules/update/src/Hook/UpdateHooks.php
+++ b/core/modules/update/src/Hook/UpdateHooks.php
    $langcode = $message['langcode'];
    $language = \Drupal::languageManager()->getLanguage($langcode);
-    $message['subject'] .= $this->t('New release(s) available for @site_name', ['@site_name' => \Drupal::config('system.site')->get('name')], ['langcode' => $langcode]);
+
+    // Set the message subject.
+    $subject = '';
+    $all_reasons = array_unique(array_values($params));
+    // If any of the update reasons in $params are security-related, then use
+    // the security subject.
+    $security = array_filter($all_reasons, function ($item) {
+      return in_array($item, [UpdateManagerInterface::NOT_SECURE, UpdateManagerInterface::REVOKED]);
+    });
+    if ($security) {
+      $subject = $this->t('Security release(s) available for @site_name', [
+        '@site_name' => \Drupal::config('system.site')
+          ->get('name'),
-        '@site_name' => \Drupal::config('system.site')
-          ->get('name'),
+        '@site_name' => \Drupal::config('system.site')->get('name'),
--- a/core/modules/update/src/Hook/UpdateHooks.php
+++ b/core/modules/update/src/Hook/UpdateHooks.php
+        '@site_name' => \Drupal::config('system.site')
+          ->get('name'),
+        'langcode' => $langcode,
+      ]);
+    }
+
+    // If security updates are not included, check next for fetch failures. If
+    // all the reasons are fetch failures, then use that as the subject.
+    if (empty($subject)) {
+      $fetch = array_filter($all_reasons, function ($item) {
+        return $item > 0;
+      });
+      if (empty($fetch) && !empty($all_reasons)) {
+        $subject = $this->t('Failed to get release information for @site_name', [
+          '@site_name' => \Drupal::config('system.site')
+            ->get('name'),
-          '@site_name' => \Drupal::config('system.site')
-            ->get('name'),
+          '@site_name' => \Drupal::config('system.site')->get('name'),
--- a/core/modules/update/src/Hook/UpdateHooks.php
+++ b/core/modules/update/src/Hook/UpdateHooks.php
+      });
+      if (empty($fetch) && !empty($all_reasons)) {
+        $subject = $this->t('Failed to get release information for @site_name', [
+          '@site_name' => \Drupal::config('system.site')
+            ->get('name'),
+          'langcode' => $langcode,
+        ]);
+      }
+    }
+
+    // If the subject still has not been set, then there are only non-security
+    // updates. Use the subject for non-security updates.
+    if (empty($subject)) {
+      $subject = $this->t('New release(s) available for @site_name', [
+        '@site_name' => \Drupal::config('system.site')
+          ->get('name'),
-        '@site_name' => \Drupal::config('system.site')
-          ->get('name'),
+        '@site_name' => \Drupal::config('system.site')->get('name'),
--- a/core/modules/update/tests/src/Unit/UpdateMailTest.php
+++ b/core/modules/update/tests/src/Unit/UpdateMailTest.php
+    for ($i = 0; $i < count($expected_body); $i++) {
+      is_string($message['body'][$i]) ? $this->assertSame($expected_body[$i], $message['body'][$i]) : $this->assertSame($expected_body[$i], $message['body'][$i]->render());
    }
-    else {
-      if (empty($params)) {
-        $this->assertSame($expected_body[0], $message['body'][0]);
-        $this->assertSame($expected_body[1], $message['body'][1]->render());
-      }
-      else {
-        $this->assertSame($expected_body[0], $message['body'][0]->render());
-        $this->assertSame($expected_body[1], $message['body'][1]);
-        $this->assertSame($expected_body[2], $message['body'][2]);
-        $this->assertSame($expected_body[3], $message['body'][3]->render());
-      }
+    for ($i = 0; $i < count($expected_body); $i++) {
+      is_string($message['body'][$i]) ? $this->assertSame($expected_body[$i], $message['body'][$i]) : $this->assertSame($expected_body[$i], $message['body'][$i]->render());
--- a/core/modules/update/tests/src/Unit/UpdateMailTest.php
+++ b/core/modules/update/tests/src/Unit/UpdateMailTest.php
-        ],
+        "New release(s) available for ",
+      [
+        "See the available updates page for more information:\nhttps://example.com/admin/reports/updates/settings",
+        'Your site is currently configured to send these emails when any updates are available. To get notified only for security updates, https://example.com/admin/reports/updates.',
+      ],
      ],
      'security' => [
        'security',
        [],
        FALSE,
+        "New release(s) available for ",
        [
          "See the available updates page for more information:\nhttps://example.com/admin/reports/updates/settings",
-          'Your site is currently configured to send these emails only when security updates are available. To get notified for any available updates, https://example.com/admin/reports/updates.',
+          "Your site is currently configured to send these emails only when security updates are available. To get notified for any available updates, https://example.com/admin/reports/updates.",
--- a/core/modules/update/tests/src/Unit/UpdateMailTest.php
+++ b/core/modules/update/tests/src/Unit/UpdateMailTest.php
+      ],
+      'all fetch fail' => [
+        'all',
+        [
+          'core' => UpdateFetcherInterface::NOT_FETCHED,
+          'contrib' => UpdateFetcherInterface::NOT_FETCHED,
+        ],
+        FALSE,
+        "Failed to get release information for ",
+        [
+          'There was a problem checking <a href="https://example.com/admin/reports/updates/settings">available updates</a> for Drupal.',
+          'There was a problem checking <a href="https://example.com/admin/reports/updates/settings">available updates</a> for your modules or themes.',
+          "See the available updates page for more information:\nhttps://example.com/admin/reports/updates/settings",
+          "Your site is currently configured to send these emails when any updates are available. To get notified only for security updates, https://example.com/admin/reports/updates.",
+        ],
+      ],
--- a/core/modules/update/src/Hook/UpdateHooks.php
+++ b/core/modules/update/src/Hook/UpdateHooks.php
+            ->get('name'),
+          'langcode' => $langcode,
+        ]);
+      }
+    }
+
+    // If the subject still has not been set, then there are only non-security
+    // updates. Use the subject for non-security updates.
+    if (empty($subject)) {
+      $subject = $this->t('New release(s) available for @site_name', [
+        '@site_name' => \Drupal::config('system.site')
+          ->get('name'),
+        'langcode' => $langcode,
+      ]);
+    }
+    // Update the existing message.
-    // Update the existing message.
+
+    // Append the update message to the existing subject.