diff --git a/core/lib/Drupal/Component/Utility/Unicode.php b/core/lib/Drupal/Component/Utility/Unicode.php
index 80861494f9..d11e2101f4 100644
--- a/core/lib/Drupal/Component/Utility/Unicode.php
+++ b/core/lib/Drupal/Component/Utility/Unicode.php
@@ -603,13 +603,13 @@ public static function strcasecmp($str1 , $str2) {
    *
    * @param string $string
    *   The header to encode.
-   * @param bool $first_chunk_only
+   * @param bool $shorten
    *   If TRUE, only return the first chunk of a multi-chunk encoded string.
    *
    * @return string
    *   The mime-encoded header.
    */
-  public static function mimeHeaderEncode($string, $first_chunk_only = FALSE) {
+  public static function mimeHeaderEncode($string, $shorten = FALSE) {
     if (preg_match('/[^\x20-\x7E]/', $string)) {
       $chunk_size = 47; // floor((75 - strlen("=?UTF-8?B??=")) * 0.75);
       $len = strlen($string);
@@ -617,7 +617,7 @@ public static function mimeHeaderEncode($string, $first_chunk_only = FALSE) {
       while ($len > 0) {
         $chunk = static::truncateBytes($string, $chunk_size);
         $output .= ' =?UTF-8?B?' . base64_encode($chunk) . "?=\n";
-        if ($first_chunk_only) {
+        if ($shorten) {
           break;
         }
         $c = strlen($chunk);
diff --git a/core/modules/system/tests/src/Functional/Mail/MailTest.php b/core/modules/system/tests/src/Functional/Mail/MailTest.php
index b39707111f..e8accddb6f 100644
--- a/core/modules/system/tests/src/Functional/Mail/MailTest.php
+++ b/core/modules/system/tests/src/Functional/Mail/MailTest.php
@@ -2,6 +2,7 @@
 
 namespace Drupal\Tests\system\Functional\Mail;
 
+use Drupal\Component\Utility\Unicode;
 use Drupal\Core\Mail\Plugin\Mail\TestMailCollector;
 use Drupal\Tests\BrowserTestBase;
 use Drupal\system_mail_failure_test\Plugin\Mail\TestPhpMailFailure;
@@ -90,12 +91,15 @@ public function testFromAndReplyToHeader() {
     $this->assertEqual($reply_email, $sent_message['headers']['Reply-to'], 'Message reply-to headers are set.');
     $this->assertFalse(isset($sent_message['headers']['Errors-To']), 'Errors-to header must not be set, it is deprecated.');
 
+    // Test that long site names containing characters that need MIME encoding
+    // works as expected.
     $this->config('system.site')->set('name', 'Drépal this is a very long test sentence to test what happens with very long site names')->save();
     // Send an email and check that the From-header contains the site name.
     \Drupal::service('plugin.manager.mail')->mail('simpletest', 'from_test', 'from_test@example.com', $language);
     $captured_emails = \Drupal::state()->get('system.test_mail_collector');
     $sent_message = end($captured_emails);
-    $this->assertEqual('=?UTF-8?B?RHLDqXBhbCB0aGlzIGlzIGEgdmVyeSBsb25nIHRlc3Qgc2VudGVuY2UgdG8gdGU=?= <simpletest@example.com>', $sent_message['headers']['From'], 'From header is correctly encoded.');
+    $this->assertEquals('=?UTF-8?B?RHLDqXBhbCB0aGlzIGlzIGEgdmVyeSBsb25nIHRlc3Qgc2VudGVuY2UgdG8gdGU=?= <simpletest@example.com>', $sent_message['headers']['From'], 'From header is correctly encoded.');
+    $this->assertEquals('Drépal this is a very long test sentence to te <simpletest@example.com>', Unicode::mimeHeaderDecode($sent_message['headers']['From']), 'From header is correctly encoded.');
     $this->assertFalse(isset($sent_message['headers']['Reply-to']), 'Message reply-to is not set if not specified.');
     $this->assertFalse(isset($sent_message['headers']['Errors-To']), 'Errors-to header must not be set, it is deprecated.');
   }