diff --git a/src/Symfony/Component/Mime/Tests/AddressTest.php b/src/Symfony/Component/Mime/Tests/AddressTest.php
index fe10c73910bde..5e7bc6105df7c 100644
--- a/src/Symfony/Component/Mime/Tests/AddressTest.php
+++ b/src/Symfony/Component/Mime/Tests/AddressTest.php
@@ -44,6 +44,14 @@ public function testCreate()
         $this->assertEquals($a, Address::create('fabien@symfony.com'));
     }
 
+    public function testCreateWithInvalidFormat()
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('Could not parse "<fabien@symfony" to a "Symfony\Component\Mime\Address" instance.');
+
+        Address::create('<fabien@symfony');
+    }
+
     /**
      * @dataProvider fromStringProvider
      */
diff --git a/src/Symfony/Component/Mime/Tests/Encoder/IdnAddressEncoderTest.php b/src/Symfony/Component/Mime/Tests/Encoder/IdnAddressEncoderTest.php
index 656225aa4db24..8d3cb45cb9a03 100644
--- a/src/Symfony/Component/Mime/Tests/Encoder/IdnAddressEncoderTest.php
+++ b/src/Symfony/Component/Mime/Tests/Encoder/IdnAddressEncoderTest.php
@@ -9,9 +9,10 @@
  * file that was distributed with this source code.
  */
 
-namespace Symfony\Component\Mime\Encoder;
+namespace Symfony\Component\Mime\Tests\Encoder;
 
 use PHPUnit\Framework\TestCase;
+use Symfony\Component\Mime\Encoder\IdnAddressEncoder;
 
 class IdnAddressEncoderTest extends TestCase
 {
diff --git a/src/Symfony/Component/Mime/Tests/Encoder/QpContentEncoderTest.php b/src/Symfony/Component/Mime/Tests/Encoder/QpContentEncoderTest.php
new file mode 100644
index 0000000000000..750c74f1d461a
--- /dev/null
+++ b/src/Symfony/Component/Mime/Tests/Encoder/QpContentEncoderTest.php
@@ -0,0 +1,26 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Mime\Tests\Encoder;
+
+use PHPUnit\Framework\TestCase;
+use Symfony\Component\Mime\Encoder\QpContentEncoder;
+
+class QpContentEncoderTest extends TestCase
+{
+    public function testReplaceLastChar()
+    {
+        $encoder = new QpContentEncoder();
+
+        $this->assertSame('message=09', $encoder->encodeString('message'.chr(0x09)));
+        $this->assertSame('message=20', $encoder->encodeString('message'.chr(0x20)));
+    }
+}
diff --git a/src/Symfony/Component/Mime/Tests/MessageTest.php b/src/Symfony/Component/Mime/Tests/MessageTest.php
index 6d981e7eb1f6d..9f5fc1f713ac7 100644
--- a/src/Symfony/Component/Mime/Tests/MessageTest.php
+++ b/src/Symfony/Component/Mime/Tests/MessageTest.php
@@ -134,6 +134,16 @@ public function testGenerateMessageIdThrowsWhenHasFromButNoAddresses()
         $message->generateMessageId();
     }
 
+    public function testGenerateMessageIdThrowsWhenNeitherFromNorSenderIsPresent()
+    {
+        $message = new Message();
+
+        $this->expectException(LogicException::class);
+        $this->expectExceptionMessage('An email must have a "From" or a "Sender" header.');
+
+        $message->generateMessageId();
+    }
+
     public function testToString()
     {
         $message = new Message();
diff --git a/src/Symfony/Component/Mime/Tests/Part/Multipart/FormDataPartTest.php b/src/Symfony/Component/Mime/Tests/Part/Multipart/FormDataPartTest.php
index d86c5f84da131..22e38c5db235e 100644
--- a/src/Symfony/Component/Mime/Tests/Part/Multipart/FormDataPartTest.php
+++ b/src/Symfony/Component/Mime/Tests/Part/Multipart/FormDataPartTest.php
@@ -228,4 +228,12 @@ public function testBoundaryContentTypeHeader()
             $headers[0]
         );
     }
+
+    public function testConstructThrowsOnUnexpectedFieldType()
+    {
+        $this->expectException(InvalidArgumentException::class);
+        $this->expectExceptionMessage('A form field value can only be a string, an array, or an instance of TextPart ("stdClass" given).');
+
+        new FormDataPart(['foo' => new \stdClass()]);
+    }
 }
diff --git a/src/Symfony/Component/Mime/Tests/Test/Constraint/EmailAddressContainsTest.php b/src/Symfony/Component/Mime/Tests/Test/Constraint/EmailAddressContainsTest.php
new file mode 100644
index 0000000000000..227a51f58a4b7
--- /dev/null
+++ b/src/Symfony/Component/Mime/Tests/Test/Constraint/EmailAddressContainsTest.php
@@ -0,0 +1,40 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Mime\Tests\Test\Constraint;
+
+use PHPUnit\Framework\ExpectationFailedException;
+use PHPUnit\Framework\TestCase;
+use Symfony\Component\Mime\Email;
+use Symfony\Component\Mime\Header\Headers;
+use Symfony\Component\Mime\Test\Constraint\EmailAddressContains;
+
+class EmailAddressContainsTest extends TestCase
+{
+    public function testToString()
+    {
+        $constraint = new EmailAddressContains('headerName', 'expectedValue');
+
+        $this->assertSame('contains address "headerName" with value "expectedValue"', $constraint->toString());
+    }
+
+    public function testFailureDescription()
+    {
+        $mailboxHeader = 'text@example.com';
+        $headers = new Headers();
+        $headers->addMailboxHeader($mailboxHeader, 'actualValue@example.com');
+
+        $this->expectException(ExpectationFailedException::class);
+        $this->expectExceptionMessage('Failed asserting that the Email contains address "text@example.com" with value "expectedValue@example.com" (value is actualValue@example.com).');
+
+        (new EmailAddressContains($mailboxHeader, 'expectedValue@example.com'))->evaluate(new Email($headers));
+    }
+}
diff --git a/src/Symfony/Component/Mime/Tests/Test/Constraint/EmailAttachmentCountTest.php b/src/Symfony/Component/Mime/Tests/Test/Constraint/EmailAttachmentCountTest.php
new file mode 100644
index 0000000000000..60976675ab3d5
--- /dev/null
+++ b/src/Symfony/Component/Mime/Tests/Test/Constraint/EmailAttachmentCountTest.php
@@ -0,0 +1,38 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Mime\Tests\Test\Constraint;
+
+use PHPUnit\Framework\ExpectationFailedException;
+use PHPUnit\Framework\TestCase;
+use Symfony\Component\Mime\Email;
+use Symfony\Component\Mime\Test\Constraint\EmailAttachmentCount;
+
+class EmailAttachmentCountTest extends TestCase
+{
+    public function testToString()
+    {
+        $constraint = new EmailAttachmentCount(1);
+
+        $this->assertSame('has sent "1" attachment(s)', $constraint->toString());
+    }
+
+    public function testFailureDescription()
+    {
+        $email = new Email();
+        $email->attach('attachment content', 'attachment.txt');
+
+        $this->expectException(ExpectationFailedException::class);
+        $this->expectExceptionMessage('Failed asserting that the Email has sent "2" attachment(s).');
+
+        (new EmailAttachmentCount(2))->evaluate($email);
+    }
+}
diff --git a/src/Symfony/Component/Mime/Tests/Test/Constraint/EmailHasHeaderTest.php b/src/Symfony/Component/Mime/Tests/Test/Constraint/EmailHasHeaderTest.php
new file mode 100644
index 0000000000000..ae5f75fddfb55
--- /dev/null
+++ b/src/Symfony/Component/Mime/Tests/Test/Constraint/EmailHasHeaderTest.php
@@ -0,0 +1,39 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Mime\Tests\Test\Constraint;
+
+use PHPUnit\Framework\ExpectationFailedException;
+use PHPUnit\Framework\TestCase;
+use Symfony\Component\Mime\Email;
+use Symfony\Component\Mime\Header\Headers;
+use Symfony\Component\Mime\Test\Constraint\EmailHasHeader;
+
+class EmailHasHeaderTest extends TestCase
+{
+    public function testToString()
+    {
+        $constraint = new EmailHasHeader('headerName');
+
+        $this->assertSame('has header "headerName"', $constraint->toString());
+    }
+
+    public function testFailureDescription()
+    {
+        $headers = new Headers();
+        $headers->addMailboxHeader('headerName', 'test@example.com');
+
+        $this->expectException(ExpectationFailedException::class);
+        $this->expectExceptionMessage('Failed asserting that the Email has header "not existing header".');
+
+        (new EmailHasHeader('not existing header'))->evaluate(new Email($headers));
+    }
+}
diff --git a/src/Symfony/Component/Mime/Tests/Test/Constraint/EmailHtmlBodyContainsTest.php b/src/Symfony/Component/Mime/Tests/Test/Constraint/EmailHtmlBodyContainsTest.php
new file mode 100644
index 0000000000000..ae994b0959796
--- /dev/null
+++ b/src/Symfony/Component/Mime/Tests/Test/Constraint/EmailHtmlBodyContainsTest.php
@@ -0,0 +1,39 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Mime\Tests\Test\Constraint;
+
+use PHPUnit\Framework\ExpectationFailedException;
+use PHPUnit\Framework\TestCase;
+use Symfony\Component\Mime\Email;
+use Symfony\Component\Mime\Test\Constraint\EmailHtmlBodyContains;
+
+class EmailHtmlBodyContainsTest extends TestCase
+{
+    public function testToString()
+    {
+        $constraint = new EmailHtmlBodyContains('expectedValue');
+
+        $this->assertSame('contains "expectedValue"', $constraint->toString());
+    }
+
+    public function testFailureDescription()
+    {
+        $expectedValue = 'expectedValue';
+        $email = new Email();
+        $email->html('actualValue')->text($expectedValue);
+
+        $this->expectException(ExpectationFailedException::class);
+        $this->expectExceptionMessage('Failed asserting that the Email HTML body contains "expectedValue".');
+
+        (new EmailHtmlBodyContains($expectedValue))->evaluate($email);
+    }
+}
diff --git a/src/Symfony/Component/Mime/Tests/Test/Constraint/EmailTextBodyContainsTest.php b/src/Symfony/Component/Mime/Tests/Test/Constraint/EmailTextBodyContainsTest.php
new file mode 100644
index 0000000000000..43ba0170ef52c
--- /dev/null
+++ b/src/Symfony/Component/Mime/Tests/Test/Constraint/EmailTextBodyContainsTest.php
@@ -0,0 +1,39 @@
+<?php
+
+/*
+ * This file is part of the Symfony package.
+ *
+ * (c) Fabien Potencier <fabien@symfony.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+namespace Symfony\Component\Mime\Tests\Test\Constraint;
+
+use PHPUnit\Framework\ExpectationFailedException;
+use PHPUnit\Framework\TestCase;
+use Symfony\Component\Mime\Email;
+use Symfony\Component\Mime\Test\Constraint\EmailTextBodyContains;
+
+class EmailTextBodyContainsTest extends TestCase
+{
+    public function testToString()
+    {
+        $constraint = new EmailTextBodyContains('expectedValue');
+
+        $this->assertSame('contains "expectedValue"', $constraint->toString());
+    }
+
+    public function testFailureDescription()
+    {
+        $expectedValue = 'expectedValue';
+        $email = new Email();
+        $email->html($expectedValue)->text('actualValue');
+
+        $this->expectException(ExpectationFailedException::class);
+        $this->expectExceptionMessage('Failed asserting that the Email text body contains "expectedValue".');
+
+        (new EmailTextBodyContains($expectedValue))->evaluate($email);
+    }
+}