diff --git a/core/includes/mail.inc b/core/includes/mail.inc
index 5694a2e..afc206c 100644
--- a/core/includes/mail.inc
+++ b/core/includes/mail.inc
@@ -217,7 +217,7 @@ function drupal_mail($module, $key, $to, $langcode, $params = array(), $from = N
  *   Drupal\Core\Mail\PhpMail implementation.
  *
  * The selection of a particular implementation is controlled via the config
- * 'system.mail.system', which is a keyed array.  The default implementation
+ * 'system.mail.interface', which is a keyed array.  The default implementation
  * is the class whose name is the value of 'default' key. A more specific match
  * first to key and then to module will be used in preference to the default. To
  * specify a different class for all mail sent by one module, set the class
@@ -268,7 +268,7 @@ function drupal_mail_system($module, $key) {
 
   $id = $module . '_' . $key;
 
-  $configuration = config('system.mail')->get('default');
+  $configuration = config('system.mail')->get('interface');
 
   // Look for overrides for the default class, starting from the most specific
   // id, and falling back to the module name.
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
index ae8dddf..65de619 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
@@ -835,7 +835,7 @@ protected function setUp() {
     $this->resetAll();
 
     // Use the test mail class instead of the default mail handler class.
-    config('system.mail')->set('default', 'Drupal\Core\Mail\VariableLog')->save();
+    config('system.mail')->set('interface.default', 'Drupal\Core\Mail\VariableLog')->save();
 
     drupal_set_time_limit($this->timeLimit);
     // Temporary fix so that when running from run-tests.sh we don't get an
diff --git a/core/modules/system/config/system.mail.yml b/core/modules/system/config/system.mail.yml
index d245a2a..e3eb768 100644
--- a/core/modules/system/config/system.mail.yml
+++ b/core/modules/system/config/system.mail.yml
@@ -1 +1,2 @@
-default: 'Drupal\Core\Mail\PhpMail'
+interface:
+ default: 'Drupal\Core\Mail\PhpMail'
diff --git a/core/modules/system/lib/Drupal/system/Tests/Mail/MailTest.php b/core/modules/system/lib/Drupal/system/Tests/Mail/MailTest.php
index fa026c0..155fdb8 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Mail/MailTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Mail/MailTest.php
@@ -42,7 +42,7 @@ function setUp() {
     parent::setUp();
 
     // Set MailTestCase (i.e. this class) as the SMTP library
-    config('system.mail')->set('default', 'Drupal\system\Tests\Mail\MailTest')->save();
+    config('system.mail')->set('interface.default', 'Drupal\system\Tests\Mail\MailTest')->save();
   }
 
   /**
diff --git a/core/modules/system/system.install b/core/modules/system/system.install
index 32f4181..ff27859 100644
--- a/core/modules/system/system.install
+++ b/core/modules/system/system.install
@@ -2220,7 +2220,7 @@ function system_update_8046() {
  */
 function system_update_8047() {
   update_variables_to_config('system.mail', array(
-    'mail_system' => 'default',
+    'mail_system' => 'interface.default',
   ));
 }