Description:
------------
When sending a long plain-text message by mail(), single characters at specific positions are omitted. The interval between missing characters is about 810 characters. It is independent of text sent, formatting, line-ending commands or number of characters in one row.
PHP: 4.3.3 as a module of Apache 2.0.47, Win XP Prof. CZ,
Windows firewall OFF, standard SMTP server.

SMTP-related changes in php.ini:
SMTP = mailgw.cvut.cz
sendmail_from = sistekj@vc.cvut.cz

Reproduce code:
---------------
<?php
$long_row="Text to be repeated many times, any data, any contents, anything can be here. This sentence is to fill the space  \r\n ";

$message="";
for ($i = 1; $i <= 80; $i++) {
    $message=$message.$long_row; //very looong message body
}
mail("sistekj@vc.cvut.cz", "Test message", $message, "From: sistekj@vc.cvut.cz");
?>

Expected result:
----------------
$long_row repeated 80 times.