diff --git a/core/lib/Drupal/Component/Utility/Unicode.php b/core/lib/Drupal/Component/Utility/Unicode.php
index 75ffa1b..923edca 100644
--- a/core/lib/Drupal/Component/Utility/Unicode.php
+++ b/core/lib/Drupal/Component/Utility/Unicode.php
@@ -569,8 +569,7 @@ public static function strcasecmp($str1 , $str2) {
    * - Only encode strings that contain non-ASCII characters.
    * - We progressively cut-off a chunk with self::truncateBytes(). This ensures
    *   each chunk starts and ends on a character boundary.
-   * - Using \n as the chunk separator may cause problems on some systems and
-   *   may have to be changed to \r\n or \r.
+   * - According to the RFC 2047, we use \r\n as the chunk separator.
    *
    * @param string $string
    *   The header to encode.
@@ -580,12 +579,12 @@ public static function strcasecmp($str1 , $str2) {
    */
   public static function mimeHeaderEncode($string) {
     if (preg_match('/[^\x20-\x7E]/', $string)) {
-      $chunk_size = 47; // floor((75 - strlen("=?UTF-8?B??=")) * 0.75);
+      $chunk_size = 45; // floor((75 - strlen("=?UTF-8?B??=\r\n ")) * 0.75);
       $len = strlen($string);
       $output = '';
       while ($len > 0) {
         $chunk = static::truncateBytes($string, $chunk_size);
-        $output .= ' =?UTF-8?B?' . base64_encode($chunk) . "?=\n";
+        $output .= ' =?UTF-8?B?' . base64_encode($chunk) . "?=\r\n ";
         $c = strlen($chunk);
         $string = substr($string, $c);
         $len -= $c;
diff --git a/core/tests/Drupal/Tests/Component/Utility/UnicodeTest.php b/core/tests/Drupal/Tests/Component/Utility/UnicodeTest.php
index 724d875..1a8a08a 100644
--- a/core/tests/Drupal/Tests/Component/Utility/UnicodeTest.php
+++ b/core/tests/Drupal/Tests/Component/Utility/UnicodeTest.php
@@ -90,8 +90,14 @@ public function testMimeHeader($value, $encoded) {
   public function providerTestMimeHeader() {
     return array(
       array('tést.txt', '=?UTF-8?B?dMOpc3QudHh0?='),
+      // String longer than 47 characters including special characters. The
+      // result is an encoded chunk splitted in two parts separated by a CLRF
+      // and a space.
+      array('aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaà', "=?UTF-8?B?YWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFhYWFh?=\r\n  =?UTF-8?B?YWHDoA==?="),
       // Simple ASCII characters.
       array('ASCII', 'ASCII'),
+      // Long ASCII string (more than 47 characters).
+      array('aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa', 'aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa'),
     );
   }