diff --git a/framework/CHANGELOG.md b/framework/CHANGELOG.md
index 6424becaa7..1fa3646e0f 100644
--- a/framework/CHANGELOG.md
+++ b/framework/CHANGELOG.md
@@ -24,6 +24,7 @@ Yii Framework 2 Change Log
 - Enh #18904: Improve Captcha client-side validation (hexkir)
 - Bug #18913: Add filename validation for `MessageSource::getMessageFilePath()` (uaoleg)
 - Bug #18909: Fix bug with binding default action parameters for controllers (bizley)
+- Bug #18955: Check `yiisoft/yii2-swiftmailer` before using as default mailer in `yii\base\Application` (WinterSilence)
 
 
 2.0.43 August 09, 2021
diff --git a/framework/base/Application.php b/framework/base/Application.php
index 61268f5bf8..3e095447c1 100644
--- a/framework/base/Application.php
+++ b/framework/base/Application.php
@@ -589,6 +589,7 @@ abstract class Application extends Module
     /**
      * Returns the mailer component.
      * @return \yii\mail\MailerInterface the mailer application component.
+     * @throws InvalidConfigException If this component is not configured.
      */
     public function getMailer()
     {
@@ -597,8 +598,7 @@ abstract class Application extends Module
 
     /**
      * Returns the auth manager for this application.
-     * @return \yii\rbac\ManagerInterface the auth manager application component.
-     * Null is returned if auth manager is not configured.
+     * @return \yii\rbac\ManagerInterface|null the auth manager application component or null if it's not configured.
      */
     public function getAuthManager()
     {
@@ -625,20 +625,25 @@ abstract class Application extends Module
 
     /**
      * Returns the configuration of core application components.
+     * @return array
      * @see set()
      */
     public function coreComponents()
     {
-        return [
+        $components = [
             'log' => ['class' => 'yii\log\Dispatcher'],
             'view' => ['class' => 'yii\web\View'],
             'formatter' => ['class' => 'yii\i18n\Formatter'],
             'i18n' => ['class' => 'yii\i18n\I18N'],
-            'mailer' => ['class' => 'yii\swiftmailer\Mailer'],
             'urlManager' => ['class' => 'yii\web\UrlManager'],
             'assetManager' => ['class' => 'yii\web\AssetManager'],
             'security' => ['class' => 'yii\base\Security'],
         ];
+        if (class_exists('yii\swiftmailer\Mailer')) {
+            $components['mailer'] = ['class' => 'yii\swiftmailer\Mailer'];
+        }
+
+        return $components;
     }
 
     /**