diff --git a/core/modules/system/tests/src/Kernel/Mail/MailTest.php b/core/modules/system/tests/src/Kernel/Mail/MailTest.php
index c03fe92..d441792 100644
--- a/core/modules/system/tests/src/Kernel/Mail/MailTest.php
+++ b/core/modules/system/tests/src/Kernel/Mail/MailTest.php
@@ -48,7 +48,10 @@ protected function setUp() {
    */
   public function testPluggableFramework() {
     // Switch mail backends.
-    $this->config('system.mail')->set('interface.default', 'test_php_mail_failure')->save();
+    // KernelTestBase enforces the usage of 'test_mail_collector' plugin to
+    // collect mail. Since we need to test this functionality itself, we
+    // manually configure the default mail interface.
+    $GLOBALS['config']['system.mail']['interface']['default'] = 'test_php_mail_failure';
 
     // Get the default MailInterface class instance.
     $mail_backend = \Drupal::service('plugin.manager.mail')->getInstance(['module' => 'default', 'key' => 'default']);
@@ -75,7 +78,11 @@ public function testPluggableFramework() {
    */
   public function testErrorMessageDisplay() {
     // Switch mail backends.
-    $this->config('system.mail')->set('interface.default', 'test_php_mail_failure')->save();
+    // KernelTestBase enforces the usage of 'test_mail_collector' plugin to
+    // collect mail. Since we need to test this functionality itself, we
+    // manually configure the default mail interface.
+    $GLOBALS['config']['system.mail']['interface']['default'] = 'test_php_mail_failure';
+
 
     // Test with errors displayed to users.
     \Drupal::service('plugin.manager.mail')->mail('default', 'default', 'test@example.com', 'en');
@@ -190,7 +197,10 @@ public function testFromAndReplyToHeader() {
   public function testConvertRelativeUrlsIntoAbsolute() {
     $language_interface = \Drupal::languageManager()->getCurrentLanguage();
 
-    $this->config('system.mail')->set('interface.default', 'test_html_mail_collector')->save();
+    // KernelTestBase enforces the usage of 'test_mail_collector' plugin to
+    // collect mail. Since we need to test this functionality itself, we
+    // manually configure the default mail interface.
+    $GLOBALS['config']['system.mail']['interface']['default'] = 'test_html_mail_collector';
 
     // Fetch the hostname and port for matching against.
     $http_host = \Drupal::request()->getSchemeAndHttpHost();
@@ -277,7 +287,10 @@ public function testConvertRelativeUrlsIntoAbsolute() {
   public function testRenderedElementsUseAbsolutePaths() {
     $language_interface = \Drupal::languageManager()->getCurrentLanguage();
 
-    $this->config('system.mail')->set('interface.default', 'test_html_mail_collector')->save();
+    // KernelTestBase enforces the usage of 'test_mail_collector' plugin to
+    // collect mail. Since we need to test this functionality itself, we
+    // manually configure the default mail interface.
+    $GLOBALS['config']['system.mail']['interface']['default'] = 'test_html_mail_collector';
 
     // Fetch the hostname and port for matching against.
     $http_host = \Drupal::request()->getSchemeAndHttpHost();
diff --git a/core/tests/Drupal/KernelTests/KernelTestBase.php b/core/tests/Drupal/KernelTests/KernelTestBase.php
index 5e14720..6cfbda5 100644
--- a/core/tests/Drupal/KernelTests/KernelTestBase.php
+++ b/core/tests/Drupal/KernelTests/KernelTestBase.php
@@ -384,7 +384,7 @@ private function bootKernel() {
     // tests from sending out emails and collect them in state instead.
     // While this should be enforced via settings.php prior to installation,
     // some tests expect to be able to test mail system implementations.
-    $this->config('system.mail')->set('interface.default', 'test_mail_collector')->save();
+    $GLOBALS['config']['system.mail']['interface']['default'] = 'test_mail_collector';
 
     // Manually configure the default file scheme so that modules that use file
     // functions don't have to install system and its configuration.