diff --git a/core/lib/Drupal/Component/Utility/Unicode.php b/core/lib/Drupal/Component/Utility/Unicode.php
index b222f78bc0ffdaae38eb39210f7e2819dfa6fe1e..f8e026faf69c3fb7deff6fbb482587fd068a6f6c 100644
--- a/core/lib/Drupal/Component/Utility/Unicode.php
+++ b/core/lib/Drupal/Component/Utility/Unicode.php
@@ -603,11 +603,13 @@ public static function strcasecmp($str1, $str2) {
    *
    * @param string $string
    *   The header to encode.
+   * @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) {
+  public static function mimeHeaderEncode($string, $shorten = FALSE) {
     if (preg_match('/[^\x20-\x7E]/', $string)) {
       // floor((75 - strlen("=?UTF-8?B??=")) * 0.75);
       $chunk_size = 47;
@@ -616,6 +618,9 @@ public static function mimeHeaderEncode($string) {
       while ($len > 0) {
         $chunk = static::truncateBytes($string, $chunk_size);
         $output .= ' =?UTF-8?B?' . base64_encode($chunk) . "?=\n";
+        if ($shorten) {
+          break;
+        }
         $c = strlen($chunk);
         $string = substr($string, $c);
         $len -= $c;
diff --git a/core/lib/Drupal/Core/Mail/MailManager.php b/core/lib/Drupal/Core/Mail/MailManager.php
index 6a092c9329ad3cce26c9aae8896e7021f087e1ed..348dceeb208d967356001561062e201ff26cb243 100644
--- a/core/lib/Drupal/Core/Mail/MailManager.php
+++ b/core/lib/Drupal/Core/Mail/MailManager.php
@@ -3,6 +3,7 @@
 namespace Drupal\Core\Mail;
 
 use Drupal\Component\Render\PlainTextOutput;
+use Drupal\Component\Utility\Unicode;
 use Drupal\Core\Logger\LoggerChannelFactoryInterface;
 use Drupal\Core\Plugin\DefaultPluginManager;
 use Drupal\Core\Cache\CacheBackendInterface;
@@ -248,7 +249,12 @@ public function doMail($module, $key, $to, $langcode, $params = [], $reply = NUL
     // Return-Path headers should have a domain authorized to use the
     // originating SMTP server.
     $headers['Sender'] = $headers['Return-Path'] = $site_mail;
-    $headers['From'] = $site_config->get('name') . ' <' . $site_mail . '>';
+    // Headers are usually encoded in the mail plugin that implements
+    // \Drupal\Core\Mail\MailInterface::mail(), for example,
+    // \Drupal\Core\Mail\Plugin\Mail\PhpMail::mail(). The site name must be
+    // encoded here to prevent mail plugins from encoding the email address,
+    // which would break the header.
+    $headers['From'] = Unicode::mimeHeaderEncode($site_config->get('name'), TRUE) . ' <' . $site_mail . '>';
     if ($reply) {
       $headers['Reply-to'] = $reply;
     }
diff --git a/core/modules/system/tests/src/Functional/Mail/MailTest.php b/core/modules/system/tests/src/Functional/Mail/MailTest.php
index 74549a3211e36130d3c7d1edb3be4d5a4d49eb01..e8accddb6f4e2f2bd6fbec57e6f082e1a3c4e7f0 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;
@@ -90,11 +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);
-    $this->assertEqual($from_email, $sent_message['headers']['From'], 'Message is sent from the site email account.');
+    $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.');
   }