diff -u b/core/lib/Drupal/Core/Mail/Plugin/Mail/PhpMail.php b/core/lib/Drupal/Core/Mail/Plugin/Mail/PhpMail.php
--- b/core/lib/Drupal/Core/Mail/Plugin/Mail/PhpMail.php
+++ b/core/lib/Drupal/Core/Mail/Plugin/Mail/PhpMail.php
@@ -6,6 +6,7 @@
 use Drupal\Core\Mail\MailInterface;
 use Drupal\Core\Site\Settings;
 use Symfony\Component\Mime\Header\Headers;
+use Symfony\Component\Mime\Header\UnstructuredHeader;
 
 /**
  * Defines the default Drupal mail backend, using PHP's native mail() function.
@@ -76,11 +77,12 @@
       }
     }
     $headers = new Headers();
-    $headers->addTextHeader('subject', $message['subject']);
     foreach ($message['headers'] as $name => $value) {
       $headers->addHeader($name, $value);
     }
     $line_endings = Settings::get('mail_line_endings', PHP_EOL);
+    // Prepare mail commands.
+    $mail_subject = (new UnstructuredHeader('subject', $message['subject']))->getBodyAsString();
     // Note: email uses CRLF for line-endings. PHP's API requires LF
     // on Unix and CRLF on Windows. Drupal automatically guesses the
     // line-ending format appropriate for your system. If you need to
@@ -105,7 +107,7 @@
       $additional_headers = isset($message['Return-Path']) && ($site_mail === $message['Return-Path'] || static::_isShellSafe($message['Return-Path'])) ? '-f' . $message['Return-Path'] : '';
       $mail_result = @mail(
         $message['to'],
-        $headers->get('subject')->toString(),
+        $mail_subject,
         $mail_body,
         $mail_headers,
         $additional_headers
@@ -118,7 +120,7 @@
       ini_set('sendmail_from', $message['Return-Path']);
       $mail_result = @mail(
         $message['to'],
-        $headers->get('subject')->toString(),
+        $mail_subject,
         $mail_body,
         $mail_headers
       );
only in patch2:
unchanged:
--- a/core/misc/cspell/dictionary.txt
+++ b/core/misc/cspell/dictionary.txt
@@ -933,7 +933,6 @@ middlewares
 midgardmvc
 mikey
 milli
-mimeheaders
 mimetypes
 minifyzombies
 minimatch