diff --git a/core/lib/Drupal/Component/Utility/Unicode.php b/core/lib/Drupal/Component/Utility/Unicode.php
index f344b78..fd713e4 100644
--- a/core/lib/Drupal/Component/Utility/Unicode.php
+++ b/core/lib/Drupal/Component/Utility/Unicode.php
@@ -590,7 +590,7 @@ public static function strcasecmp($str1 , $str2) {
   }
 
   /**
-   * Encodes MIME/HTTP headers that contain unencoded characters.
+   * Encodes MIME/HTTP headers containing characters that should be encoded.
    *
    * For example, Unicode::mimeHeaderEncode('tést.txt') returns
    * "=?UTF-8?B?dMOpc3QudHh0?=".
@@ -602,6 +602,13 @@ public static function strcasecmp($str1 , $str2) {
    * - We progressively cut-off a chunk with self::truncateBytes(). This ensures
    *   each chunk starts and ends on a character boundary.
    * - 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. See https://www.drupal.org/node/300387#comment-7701323
+   *   and please report details there in case of problems.
    *
    * @param string $string
    *   The header to encode.
@@ -613,9 +620,10 @@ public static function mimeHeaderEncode($string) {
     if (preg_match('/[^\x20-\x7E]/', $string)) {
       // Encoded lines must be 75 characters or less; base64 expands to 4/3
       // times the line size (in bytes), rounded up to a multiple of 4, so:
-      //    ceil($chunk_size * 4/3) must be <= 75 - strlen("=?UTF-8?B??=")
-      // => floor(($chunk_size + 2/3) * 4/3) must be <= 63
-      $chunk_size = 45; // floor((63 * 3/4) - 3/2)
+      //    ceil($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) {