diff --git a/core/includes/mail.inc b/core/includes/mail.inc
index f58a76e..4032ed7 100644
--- a/core/includes/mail.inc
+++ b/core/includes/mail.inc
@@ -569,7 +569,7 @@ function _drupal_html_to_text_pad($text, $pad, $prefix = '') {
   if (($p = strrpos($text, "\n")) === FALSE) {
     $p = -1;
   }
-  $n = max(0, 79 - (strlen($text) - $p) - strlen($prefix));
+  $n = max(0, 78 - (strlen($text) - $p) - strlen($prefix));
   // Add prefix and padding, and restore linebreak.
   return $text . $prefix . str_repeat($pad, $n) . "\n";
 }
diff --git a/core/modules/system/lib/Drupal/system/Tests/Mail/HtmlToTextTest.php b/core/modules/system/lib/Drupal/system/Tests/Mail/HtmlToTextTest.php
index 5970114..a383809 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Mail/HtmlToTextTest.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Mail/HtmlToTextTest.php
@@ -94,10 +94,10 @@ class HtmlToTextTest extends WebTestBase {
       // @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",
@@ -106,9 +106,9 @@ class HtmlToTextTest extends WebTestBase {
       '<h5>Drupal</h5><p>Drupal</p>' => "Drupal\n\nDrupal\n\n",
       '<h6>Drupal</h6>' => "Drupal\n\n",
       '<h6>Drupal</h6><p>Drupal</p>' => "Drupal\n\nDrupal\n\n",
-      '<hr />Drupal<hr />' => "------------------------------------------------------------------------------\nDrupal\n------------------------------------------------------------------------------\n",
-      '<hr/>Drupal<hr/>' => "------------------------------------------------------------------------------\nDrupal\n------------------------------------------------------------------------------\n",
-      '<hr/>Drupal<hr/><p>Drupal</p>' => "------------------------------------------------------------------------------\nDrupal\n------------------------------------------------------------------------------\nDrupal\n\n",
+      '<hr />Drupal<hr />' => "-----------------------------------------------------------------------------\nDrupal\n-----------------------------------------------------------------------------\n",
+      '<hr/>Drupal<hr/>' => "-----------------------------------------------------------------------------\nDrupal\n-----------------------------------------------------------------------------\n",
+      '<hr/>Drupal<hr/><p>Drupal</p>' => "-----------------------------------------------------------------------------\nDrupal\n-----------------------------------------------------------------------------\nDrupal\n\n",
       '<i>Drupal</i>' => "/Drupal/\n",
       '<p>Drupal</p>' => "Drupal\n\n",
       '<p>Drupal</p><p>Drupal</p>' => "Drupal\n\nDrupal\n\n",
@@ -140,7 +140,7 @@ class HtmlToTextTest extends WebTestBase {
       '<ul><li>Drupal</li><li><dl><dt>Drupal</dt><dd>Drupal</dd><dt>Drupal</dt><dd>Drupal</dd></dl></li><li>Drupal</li></ul>' => " * Drupal\n * Drupal\n       Drupal\n   Drupal\n       Drupal\n   \n * Drupal\n\n",
       // Tests malformed HTML tags.
       '<br>Drupal<br>Drupal' => "Drupal\nDrupal\n",
-      '<hr>Drupal<hr>Drupal' => "------------------------------------------------------------------------------\nDrupal\n------------------------------------------------------------------------------\nDrupal\n",
+      '<hr>Drupal<hr>Drupal' => "-----------------------------------------------------------------------------\nDrupal\n-----------------------------------------------------------------------------\nDrupal\n",
       '<ol><li>Drupal<li>Drupal</ol>' => " 1) Drupal\n 2) Drupal\n\n",
       '<ul><li>Drupal <em>Drupal</em> Drupal</ul></ul>' => " * Drupal /Drupal/ Drupal\n\n",
       '<ul><li>Drupal<li>Drupal</ol>' => " * Drupal\n * Drupal\n\n",
@@ -269,21 +269,21 @@ class HtmlToTextTest extends WebTestBase {
   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');
   }