diff -u b/src/Plugin/Mail/SwiftMailer.php b/src/Plugin/Mail/SwiftMailer.php
--- b/src/Plugin/Mail/SwiftMailer.php
+++ b/src/Plugin/Mail/SwiftMailer.php
@@ -593,7 +593,7 @@
     $message['body'] = Markup::create(implode($line_endings, array_map(function ($body) use ($applicable_format, $filter_format) {
       // If the body contains no html tags but the applicable format is HTML,
       // we can assume newlines will need be converted to <br>.
-      if ($applicable_format == SWIFTMAILER_FORMAT_HTML && strlen(strip_tags($body)) === strlen($body)) {
+      if ($applicable_format == SWIFTMAILER_FORMAT_HTML && Unicode::strlen(strip_tags($body)) === Unicode::strlen($body)) {
         // The default fallback format is 'plain_text', which escapes markup,
         // converts new lines to <br> and converts URLs to links.
         $build = [
only in patch2:
unchanged:
--- /dev/null
+++ b/tests/src/Kernel/Plugin/Mail/SwiftMailerTest.php
@@ -0,0 +1,81 @@
+<?php
+
+namespace Drupal\Tests\swiftmailer\Kernel\Plugin\Mail;
+
+use Drupal\Core\Render\Markup;
+use Drupal\KernelTests\KernelTestBase;
+use Drupal\swiftmailer\Utility\Conversion;
+
+/**
+ * @coversDefaultClass \Drupal\swiftmailer\Plugin\Mail\SwiftMailer
+ * @group swiftmailer
+ */
+class SwiftMailerTest extends KernelTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = [
+    'user',
+    'filter',
+    'swiftmailer',
+  ];
+
+  /**
+   * The swiftmailer plugin.
+   * 
+   * @var \Drupal\swiftmailer\Plugin\Mail\SwiftMailer
+   */
+  protected $plugin;
+  
+  public function setUp() {
+    parent::setUp();
+    $this->installConfig([
+      'swiftmailer',
+      'filter',
+    ]);
+    $this->installEntitySchema('user');
+    $this->installSchema('user', 'users_data');
+    $this->plugin = $this->container->get('plugin.manager.mail')
+      ->createInstance('swiftmailer');
+  }
+
+  /**
+   * Tests massaging the message body.
+   *
+   * @dataProvider bodyDataProvider
+   */
+  public function testMassageMessageBody(array $message, $expected) {
+    $message['params']['format'] = SWIFTMAILER_FORMAT_HTML;
+    $actual = $this->plugin->massageMessageBody($message);
+    $this->assertSame($expected, (string) $actual['body']);
+  }
+
+  /**
+   * Data provider of body data with markup and without HTML markup.
+   */
+  public function bodyDataProvider() {
+    return [
+      'with html' => [
+        'message' => [
+          'body' => [
+            Markup::create('<p>Lorem ipsum dolor sit amet</p>'),
+            Markup::create('<p>consetetur sadipscing elitr</p>'),
+            Markup::create('<p>sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat</p>'),
+            Markup::create('<p>sed diam voluptua.</p>'),
+          ],
+        ],
+        'expected' => "<p>Lorem ipsum dolor sit amet</p>\n<p>consetetur sadipscing elitr</p>\n<p>sed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat</p>\n<p>sed diam voluptua.</p>",
+      ],
+      'no html' => [
+        'message' => [
+          'body' => [
+            "Lorem ipsum dolor sit amet\nconsetetur sadipscing elitr\nsed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat\nsed diam voluptua.",
+          ],
+        ],
+        'expected' => "<p>Lorem ipsum dolor sit amet<br />\nconsetetur sadipscing elitr<br />\nsed diam nonumy eirmod tempor invidunt ut labore et dolore magna aliquyam erat<br />\nsed diam voluptua.</p>\n",
+      ],
+    ];
+  }
+
+}