Description:
------------
When mailing a long text without any space or such, mailing on Linux gives an unexpected exclamation mark after every 1000 chars or so. This behavoir does not occour on Windows.

Reproduce code:
---------------
<?
  $mail = "<enter_Email_Here>";
  for ($i=0;$i<10000;$i++) {
    $longtext .= "a";
  }

      $testmail = "PHP Mail Bug Test\r\n"
      ."\r\n"
      ."------_=_NextPart_PHP_SPECIAL_000\r\n"
      ."Content-Type: text/plain\r\n"
      ."\r\nPHP Mail Bug Test\r\n\r\n"
      ."------_=_NextPart_PHP_SPECIAL_000\r\n"
      ."Content-Type: text/html;\r\n"
      ."        name=\"PHP Mail Bug Test.html\"\r\n"
      ."Content-Disposition: attachment;\r\n"
      ."        filename=\"PHP Mail Bug Test.html\"\r\n\r\n"
      .$longtext
      ."\r\n\r\n------_=_NextPart_PHP_SPECIAL_000--\r\n";

      mail($mail,$mailkop,$testmail,
       "From: $mail\r\n"
      ."Reply-To: $mail\r\n"
      ."Return-Path: $mail\r\n"
      ."X-Mailer: PHP Mail Bug Test\r\n"
      ."MIME-Version: 1.0\r\n"
      ."Content-Type: multipart/mixed;\r\n"
      ."        boundary=\"----_=_NextPart_PHP_SPECIAL_000\"\r\n");

?>

Expected result:
----------------
A long line with 10000 a's

Actual result:
--------------
After 1000 chars, he gives a ! (exclamation mark) without any given reason.