diff --git a/core/lib/Drupal/Component/Utility/Unicode.php b/core/lib/Drupal/Component/Utility/Unicode.php
index f6b4668..df5bb39 100644
--- a/core/lib/Drupal/Component/Utility/Unicode.php
+++ b/core/lib/Drupal/Component/Utility/Unicode.php
@@ -585,7 +585,7 @@ public static function strcasecmp($str1 , $str2) {
   }
 
   /**
-   * Encodes MIME/HTTP headers that contain incorrectly encoded characters.
+   * Encodes MIME/HTTP headers containing characters that should be encoded.
    *
    * For example, Unicode::mimeHeaderEncode('tést.txt') returns
    * "=?UTF-8?B?dMOpc3QudHh0?=".
@@ -596,8 +596,13 @@ 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 RFC 2047, we split long lines using CLRF SPACE as separator.
+   * - PHP's mail() function (http://php.net/manual/en/function.mail.php) used
+   *   to have an explicit restriction that subject lines could not contain LF.
+   *   This restriction has been lifted since november 2009 or earlier, but
+   *   occasional problem reports have still popped up about a system not being
+   *   able to deal with "\n" in subject lines, whose source (php or mailer) was
+   *   not confirmed.
    *
    * @param string $string
    *   The header to encode.
@@ -607,16 +612,22 @@ public static function strcasecmp($str1 , $str2) {
    */
   public static function mimeHeaderEncode($string) {
     if (preg_match('/[^\x20-\x7E]/', $string)) {
-      $chunk_size = 47; // floor((75 - strlen("=?UTF-8?B??=")) * 0.75);
+      // Encoded lines must be 75 characters or less; base64 expands to 4/3
+      // times the line size (in bytes), rounded up to a multiple of 4, so:
+      //    ceil($chunk_size / 3) * 4 must be <= 75 - strlen("=?UTF-8?B??=")
+      // => ceil($chunk_size / 3) must be <= 63 / 4
+      // => $chunk_size must be <= floor(63 / 4) * 3
+      $chunk_size = 45;
       $len = strlen($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 -= $c;
       }
+      // Remove \r\n from the end of the encoded string.
       return trim($output);
     }
     return $string;
diff --git a/core/tests/Drupal/Tests/Component/Utility/UnicodeTest.php b/core/tests/Drupal/Tests/Component/Utility/UnicodeTest.php
index 39759b5..17c6fa2 100644
--- a/core/tests/Drupal/Tests/Component/Utility/UnicodeTest.php
+++ b/core/tests/Drupal/Tests/Component/Utility/UnicodeTest.php
@@ -87,6 +87,16 @@ public function providerTestMimeHeader() {
       array('tést.txt', '=?UTF-8?B?dMOpc3QudHh0?='),
       // Simple ASCII characters.
       array('ASCII', 'ASCII'),
+      // Long ASCII string (more than 45 bytes).
+      array('1234567890abcdefghij1234567890abcdefghij123456', '1234567890abcdefghij1234567890abcdefghij123456'),
+      // Strings containing non-ASCII characters:
+      // length is 44 characters, 45 bytes: converts into one encoded chunk.
+      array('1234567890abcdefghij1234567890abcdefghij123à', '=?UTF-8?B?MTIzNDU2Nzg5MGFiY2RlZmdoaWoxMjM0NTY3ODkwYWJjZGVmZ2hpajEyM8Og?='),
+      // longer than 45 bytes: converts into two encoded chunks separated by
+      // CLRF + space, according to http://www.rfc-editor.org/rfc/rfc2047.txt
+      array('1234567890abcdefghij1234567890abcdefghij1234à', "=?UTF-8?B?MTIzNDU2Nzg5MGFiY2RlZmdoaWoxMjM0NTY3ODkwYWJjZGVmZ2hpajEyMzQ=?=\r\n =?UTF-8?B?w6A=?="),
+      // ASCII string including a LF.
+      array("ASCII\nASCII", '=?UTF-8?B?QVNDSUkKQVNDSUk=?='),
     );
   }