Issue #3393337: Handle case where core mail handler mail method is triggered directly

tests/src/Kernel/SymfonyMailerKernelTest.php

@@ -75,4 +75,18 @@ class SymfonyMailerKernelTest extends KernelTestBase {
     $this->assertBodyContains('<span class="day" style="font-style: italic;">');
   }
 
+  /**
+   * Test basic core mail functionality.
+   */
+  public function testCoreMailMethod() {
+    // Get the mail manager and trigger its mail() method. This is possible in
+    // core.
+    $this->container->get('plugin.manager.mail')->mail('filter', 'key', 'admin@example.com', 'en', [
+      'subject' => 'Test Email',
+      'body' => 'This is a test.',
+    ]);
+
+    $this->assertTrue(TRUE);
+  }
+
 }