diff --git a/tests/ConfigTest.php b/tests/ConfigTest.php
index 1d7f35b..3f14f35 100644
--- a/tests/ConfigTest.php
+++ b/tests/ConfigTest.php
@@ -19,10 +19,10 @@ public function testConfig(): void
             'queueConnection' => 'foo',
         ]);
 
-        $this->assertSame('mailer-dsn', $cfg->getDSN());
-        $this->assertSame('admin@spiral.dev', $cfg->getFromAddress());
-        $this->assertSame('emails', $cfg->getQueue());
-        $this->assertSame('foo', $cfg->getQueueConnection());
+        self::assertSame('mailer-dsn', $cfg->getDSN());
+        self::assertSame('admin@spiral.dev', $cfg->getFromAddress());
+        self::assertSame('emails', $cfg->getQueue());
+        self::assertSame('foo', $cfg->getQueueConnection());
     }
 
     public function testDefaultConfig(): void
@@ -36,10 +36,10 @@ public function testDefaultConfig(): void
             'queueConnection' => $env->get('MAILER_QUEUE_CONNECTION'),
         ]);
 
-        $this->assertSame('', $config->getDSN());
-        $this->assertSame('Spiral <sendit@local.host>', $config->getFromAddress());
-        $this->assertSame('local', $config->getQueue());
-        $this->assertNull($config->getQueueConnection());
+        self::assertSame('', $config->getDSN());
+        self::assertSame('Spiral <sendit@local.host>', $config->getFromAddress());
+        self::assertSame('local', $config->getQueue());
+        self::assertNull($config->getQueueConnection());
     }
 
     public function testDefaultConfigWithQueue(): void
@@ -50,7 +50,7 @@ public function testDefaultConfigWithQueue(): void
             'queue' => $env->get('MAILER_QUEUE', 'local'),
         ]);
 
-        $this->assertSame('emails', $config->getQueue());
+        self::assertSame('emails', $config->getQueue());
     }
 
     public function testQueueWithNull(): void
@@ -59,12 +59,12 @@ public function testQueueWithNull(): void
             'queue' => null,
         ]);
 
-        $this->assertNull($config->getQueue());
+        self::assertNull($config->getQueue());
     }
 
     public function testGetsQueueConnectionWithoutKey(): void
     {
         $cfg = new MailerConfig();
-        $this->assertNull($cfg->getQueueConnection());
+        self::assertNull($cfg->getQueueConnection());
     }
 }
diff --git a/tests/JobTest.php b/tests/JobTest.php
index 66953d4..a8b6b6e 100644
--- a/tests/JobTest.php
+++ b/tests/JobTest.php
@@ -122,7 +122,7 @@ private function expectRenderer(Email $email): void
     {
         $this->renderer->expects('render')->withArgs(
             function (Message $message): bool {
-                $this->assertSame($message->getSubject(), 'test');
+                self::assertSame('test', $message->getSubject());
                 return true;
             }
         )->andReturn($email);
diff --git a/tests/JobsQueueTest.php b/tests/JobsQueueTest.php
index b0e4f47..0de8bfe 100644
--- a/tests/JobsQueueTest.php
+++ b/tests/JobsQueueTest.php
@@ -34,9 +34,9 @@ public function testQueue(): void
 
         $queue->expects('push')->withArgs(
             function ($job, $data, Options $options) use ($mail): bool {
-                $this->assertSame(MailQueue::JOB_NAME, $job);
-                $this->assertSame($data, MessageSerializer::pack($mail));
-                $this->assertSame('mailer', $options->getQueue());
+                self::assertSame(MailQueue::JOB_NAME, $job);
+                self::assertSame($data, MessageSerializer::pack($mail));
+                self::assertSame('mailer', $options->getQueue());
 
                 return true;
             }
diff --git a/tests/QueueTest.php b/tests/QueueTest.php
index 92fc215..64ba11c 100644
--- a/tests/QueueTest.php
+++ b/tests/QueueTest.php
@@ -43,10 +43,10 @@ public function testQueue(): void
 
         $this->queue->expects('push')->withArgs(
             function ($job, $data, Options $options) use ($mail): bool {
-                $this->assertSame(MailQueue::JOB_NAME, $job);
-                $this->assertSame($data, MessageSerializer::pack($mail));
-                $this->assertSame('mailer', $options->getQueue());
-                $this->assertNull($options->getDelay());
+                self::assertSame(MailQueue::JOB_NAME, $job);
+                self::assertSame($data, MessageSerializer::pack($mail));
+                self::assertSame('mailer', $options->getQueue());
+                self::assertNull($options->getDelay());
 
                 return true;
             }
@@ -68,21 +68,21 @@ public function testQueueWithDelay(): void
 
         $this->queue->expects('push')->once()->withArgs(
             function ($job, $data, Options $options): bool {
-                $this->assertSame(30, $options->getDelay());
+                self::assertSame(30, $options->getDelay());
                 return true;
             }
         );
 
         $this->queue->expects('push')->once()->withArgs(
             function ($job, $data, Options $options): bool {
-                $this->assertSame(100, $options->getDelay());
+                self::assertSame(100, $options->getDelay());
                 return true;
             }
         );
 
         $this->queue->expects('push')->once()->withArgs(
             function ($job, $data, Options $options): bool {
-                $this->assertSame(200, $options->getDelay());
+                self::assertSame(200, $options->getDelay());
                 return true;
             }
         );
diff --git a/tests/RenderTest.php b/tests/RenderTest.php
index bd2a9a1..91439ce 100644
--- a/tests/RenderTest.php
+++ b/tests/RenderTest.php
@@ -55,11 +55,11 @@ public function testRender(): void
     {
         $email = $this->send(new Message('email', ['email@domain.com'], ['name' => 'Antony']));
 
-        $this->assertSame('Demo Email', $email->getSubject());
+        self::assertSame('Demo Email', $email->getSubject());
 
         $body = $email->getBody()->toString();
-        $this->assertStringContainsString('bootstrap.txt', $body);
-        $this->assertStringContainsString('<p>Hello, Antony!</p>', $body);
+        self::assertStringContainsString('bootstrap.txt', $body);
+        self::assertStringContainsString('<p>Hello, Antony!</p>', $body);
     }
 
     private function send(MessageInterface $message): Email
diff --git a/tests/SerializerTest.php b/tests/SerializerTest.php
index f538975..4d063a4 100644
--- a/tests/SerializerTest.php
+++ b/tests/SerializerTest.php
@@ -21,10 +21,7 @@ public function testSerializeUnserialize(): void
 
         $data = MessageSerializer::pack($mail);
 
-        $this->assertSame(
-            ['subject', 'data', 'to', 'cc', 'bcc', 'from', 'replyTo', 'options'],
-            array_keys($data)
-        );
-        $this->assertEquals($mail, MessageSerializer::unpack($data));
+        self::assertSame(['subject', 'data', 'to', 'cc', 'bcc', 'from', 'replyTo', 'options'], array_keys($data));
+        self::assertEquals($mail, MessageSerializer::unpack($data));
     }
 }
diff --git a/tests/TransportResolverTest.php b/tests/TransportResolverTest.php
index 43d8b39..30d4c3c 100644
--- a/tests/TransportResolverTest.php
+++ b/tests/TransportResolverTest.php
@@ -22,7 +22,7 @@ public function testCanRegisterTransport(): void
         $transportResolver = new TransportResolver(new Transport([]));
 
         $transportResolver->registerTransport($transportFactory);
-        $this->assertCount(1, $transportResolver->getTransports());
+        self::assertCount(1, $transportResolver->getTransports());
     }
 
     public function testCanResolveRegisteredTransport(): void
@@ -38,7 +38,7 @@ public function testCanResolveRegisteredTransport(): void
 
         $transportResolver->registerTransport($transportFactory);
 
-        $this->assertSame($transport, $transportResolver->resolve('smtp://localhost'));
+        self::assertSame($transport, $transportResolver->resolve('smtp://localhost'));
     }
 
     public function testCanResolveRegisteredDefaultTransport(): void
@@ -52,7 +52,7 @@ public function testCanResolveRegisteredDefaultTransport(): void
 
         $transportResolver = new TransportResolver(new Transport([$transportFactory]));
 
-        $this->assertSame($transport, $transportResolver->resolve('smtp://localhost'));
+        self::assertSame($transport, $transportResolver->resolve('smtp://localhost'));
     }
 
     public function testNotRegisteredTransportShouldTrowAnException(): void