diff --git a/tests/Shared/BootstrapTest.php b/tests/Shared/BootstrapTest.php
new file mode 100644
index 0000000..d3e15a7
--- /dev/null
+++ b/tests/Shared/BootstrapTest.php
@@ -0,0 +1,89 @@
+<?php
+declare(strict_types=1);
+
+namespace Tests\Shared;
+
+use App\Shared\Bootstrap;
+use PHPUnit\Framework\TestCase;
+use Psr\Container\ContainerInterface;
+use Slim\Factory\AppFactory;
+
+#[\PHPUnit\Framework\Attributes\AllowMockObjectsWithoutExpectations]
+final class BootstrapTest extends TestCase
+{
+    /** @var array<string, string> */
+    private array $envBackup = [];
+
+    protected function setUp(): void
+    {
+        $this->envBackup = $_ENV;
+    }
+
+    protected function tearDown(): void
+    {
+        $_ENV = $this->envBackup;
+    }
+
+    public function testGetContainerReturnsPreloadedContainer(): void
+    {
+        $bootstrap = Bootstrap::create();
+        $container = new class implements ContainerInterface {
+            public function get(string $id): mixed
+            {
+                throw new \RuntimeException('Not expected');
+            }
+
+            public function has(string $id): bool
+            {
+                return false;
+            }
+        };
+
+        $this->setPrivateProperty($bootstrap, 'container', $container);
+
+        self::assertSame($container, $bootstrap->getContainer());
+        self::assertSame($container, $bootstrap->initializeInfrastructure());
+    }
+
+    public function testCreateHttpAppReturnsPreloadedApp(): void
+    {
+        $bootstrap = Bootstrap::create();
+        $app = AppFactory::create();
+
+        $this->setPrivateProperty($bootstrap, 'app', $app);
+
+        self::assertSame($app, $bootstrap->createHttpApp());
+    }
+
+    public function testInitializeReturnsPreloadedAppWhenAutoProvisioningDisabled(): void
+    {
+        $_ENV['APP_ENV'] = 'production';
+        $_ENV['APP_AUTO_PROVISION'] = '0';
+
+        $bootstrap = Bootstrap::create();
+        $container = new class implements ContainerInterface {
+            public function get(string $id): mixed
+            {
+                throw new \RuntimeException('Not expected');
+            }
+
+            public function has(string $id): bool
+            {
+                return false;
+            }
+        };
+        $app = AppFactory::create();
+
+        $this->setPrivateProperty($bootstrap, 'container', $container);
+        $this->setPrivateProperty($bootstrap, 'app', $app);
+
+        self::assertSame($app, $bootstrap->initialize());
+    }
+
+    private function setPrivateProperty(object $object, string $property, mixed $value): void
+    {
+        $reflection = new \ReflectionProperty($object, $property);
+        $reflection->setAccessible(true);
+        $reflection->setValue($object, $value);
+    }
+}
diff --git a/tests/Shared/MailServiceTest.php b/tests/Shared/MailServiceTest.php
new file mode 100644
index 0000000..1114b7f
--- /dev/null
+++ b/tests/Shared/MailServiceTest.php
@@ -0,0 +1,70 @@
+<?php
+declare(strict_types=1);
+
+namespace Tests\Shared;
+
+use App\Shared\Mail\MailService;
+use PHPMailer\PHPMailer\PHPMailer;
+use PHPUnit\Framework\TestCase;
+use Slim\Views\Twig;
+use Twig\Loader\ArrayLoader;
+
+#[\PHPUnit\Framework\Attributes\AllowMockObjectsWithoutExpectations]
+final class MailServiceTest extends TestCase
+{
+    public function testCreateMailerUsesSslConfiguration(): void
+    {
+        $service = $this->createService('ssl', 465);
+
+        /** @var PHPMailer $mailer */
+        $mailer = $this->invokeCreateMailer($service);
+
+        self::assertSame('smtp', $mailer->Mailer);
+        self::assertSame('smtp.example.test', $mailer->Host);
+        self::assertTrue($mailer->SMTPAuth);
+        self::assertSame('mailer-user', $mailer->Username);
+        self::assertSame('mailer-pass', $mailer->Password);
+        self::assertSame(PHPMailer::ENCRYPTION_SMTPS, $mailer->SMTPSecure);
+        self::assertSame(465, $mailer->Port);
+        self::assertSame(PHPMailer::CHARSET_UTF8, $mailer->CharSet);
+        self::assertSame('noreply@example.test', $mailer->From);
+        self::assertSame('Slim Blog', $mailer->FromName);
+    }
+
+    public function testCreateMailerUsesStartTlsWhenEncryptionIsNotSsl(): void
+    {
+        $service = $this->createService('tls', 587);
+
+        /** @var PHPMailer $mailer */
+        $mailer = $this->invokeCreateMailer($service);
+
+        self::assertSame(PHPMailer::ENCRYPTION_STARTTLS, $mailer->SMTPSecure);
+        self::assertSame(587, $mailer->Port);
+    }
+
+    private function createService(string $encryption, int $port): MailService
+    {
+        $twig = new Twig(new ArrayLoader([
+            'emails/test.twig' => '<p>Hello {{ name }}</p>',
+        ]));
+
+        return new MailService(
+            $twig,
+            'smtp.example.test',
+            $port,
+            'mailer-user',
+            'mailer-pass',
+            $encryption,
+            'noreply@example.test',
+            'Slim Blog',
+        );
+    }
+
+    private function invokeCreateMailer(MailService $service): mixed
+    {
+        $method = new \ReflectionMethod($service, 'createMailer');
+        $method->setAccessible(true);
+
+        return $method->invoke($service);
+    }
+}