diff --git a/core/lib/Drupal/Core/Mail/MailFormatHelper.php b/core/lib/Drupal/Core/Mail/MailFormatHelper.php
index 431c846ceec..12b956ceac2 100644
--- a/core/lib/Drupal/Core/Mail/MailFormatHelper.php
+++ b/core/lib/Drupal/Core/Mail/MailFormatHelper.php
@@ -142,8 +142,6 @@ class MailFormatHelper {
     // required).
     // Odd/even counter (tag or no tag).
     $tag = FALSE;
-    // Case conversion function.
-    $casing = NULL;
     $output = '';
     // All current indentation string chunks.
     $indent = [];
@@ -221,17 +219,14 @@ class MailFormatHelper {
           // Fancy headers.
           case 'h1':
             $indent[] = '======== ';
-            $casing = 'mb_strtoupper';
             break;
 
           case 'h2':
             $indent[] = '-------- ';
-            $casing = 'mb_strtoupper';
             break;
 
           case '/h1':
           case '/h2':
-            $casing = NULL;
             // Pad the line with dashes.
             $output = static::htmlToTextPad($output, ($tagname == '/h1') ? '=' : '-', ' ');
             array_pop($indent);
@@ -266,10 +261,6 @@ class MailFormatHelper {
 
       // See if there is something waiting to be output.
       if (isset($chunk)) {
-        // Apply any necessary case conversion.
-        if (isset($casing)) {
-          $chunk = call_user_func($casing, $chunk);
-        }
         $line_endings = Settings::get('mail_line_endings', PHP_EOL);
         // Format it and apply the current indentation.
         $output .= static::wrapMail($chunk, implode('', $indent)) . $line_endings;
diff --git a/core/modules/system/tests/src/Functional/Mail/HtmlToTextTest.php b/core/modules/system/tests/src/Functional/Mail/HtmlToTextTest.php
index 59d7e07e921..46dec8b2002 100644
--- a/core/modules/system/tests/src/Functional/Mail/HtmlToTextTest.php
+++ b/core/modules/system/tests/src/Functional/Mail/HtmlToTextTest.php
@@ -85,10 +85,10 @@ class HtmlToTextTest extends BrowserTestBase {
       // @todo The <div> tag is currently not supported.
       '<div>Drupal</div><div>Drupal</div>' => "DrupalDrupal\n",
       '<em>Drupal</em>' => "/Drupal/\n",
-      '<h1>Drupal</h1>' => "======== DRUPAL ==============================================================\n\n",
-      '<h1>Drupal</h1><p>Drupal</p>' => "======== DRUPAL ==============================================================\n\nDrupal\n\n",
-      '<h2>Drupal</h2>' => "-------- DRUPAL --------------------------------------------------------------\n\n",
-      '<h2>Drupal</h2><p>Drupal</p>' => "-------- DRUPAL --------------------------------------------------------------\n\nDrupal\n\n",
+      '<h1>Drupal</h1>' => "======== Drupal ==============================================================\n\n",
+      '<h1>Drupal</h1><p>Drupal</p>' => "======== Drupal ==============================================================\n\nDrupal\n\n",
+      '<h2>Drupal</h2>' => "-------- Drupal --------------------------------------------------------------\n\n",
+      '<h2>Drupal</h2><p>Drupal</p>' => "-------- Drupal --------------------------------------------------------------\n\nDrupal\n\n",
       '<h3>Drupal</h3>' => ".... Drupal\n\n",
       '<h3>Drupal</h3><p>Drupal</p>' => ".... Drupal\n\nDrupal\n\n",
       '<h4>Drupal</h4>' => ".. Drupal\n\n",
@@ -247,21 +247,21 @@ EOT;
   public function testHeaderSeparation() {
     $html = 'Drupal<h1>Drupal</h1>Drupal';
     // @todo There should be more space above the header than below it.
-    $text = "Drupal\n======== DRUPAL ==============================================================\n\nDrupal\n";
+    $text = "Drupal\n======== Drupal ==============================================================\n\nDrupal\n";
     $this->assertHtmlToText($html, $text,
       'Text before and after <h1> tag');
     $html = '<p>Drupal</p><h1>Drupal</h1>Drupal';
     // @todo There should be more space above the header than below it.
-    $text = "Drupal\n\n======== DRUPAL ==============================================================\n\nDrupal\n";
+    $text = "Drupal\n\n======== Drupal ==============================================================\n\nDrupal\n";
     $this->assertHtmlToText($html, $text,
       'Paragraph before and text after <h1> tag');
     $html = 'Drupal<h1>Drupal</h1><p>Drupal</p>';
     // @todo There should be more space above the header than below it.
-    $text = "Drupal\n======== DRUPAL ==============================================================\n\nDrupal\n\n";
+    $text = "Drupal\n======== Drupal ==============================================================\n\nDrupal\n\n";
     $this->assertHtmlToText($html, $text,
       'Text before and paragraph after <h1> tag');
     $html = '<p>Drupal</p><h1>Drupal</h1><p>Drupal</p>';
-    $text = "Drupal\n\n======== DRUPAL ==============================================================\n\nDrupal\n\n";
+    $text = "Drupal\n\n======== Drupal ==============================================================\n\nDrupal\n\n";
     $this->assertHtmlToText($html, $text,
       'Paragraph before and after <h1> tag');
   }