<b></b>), которых там быть не должно и не попала подпись, поэтому расхождение именно по тексту получилось довольно значительным.
$uid = uniqid('sf');
$header = "MIME-Version: 1.0\r\n";
$header .= "From: ".$FromName." <".$From.">"."\r\n";
$header .= "Reply-To: ".$FromName." <".$From.">\r\n";
$header .= "Organization: ".$dommen." \r\n";
$header .= "Content-Type: multipart/alternative; boundary=\"" . $uid . "\"\r\n";
$header .= "Date: ".date("D, j M Y G:i:s")." +0300\r\n";
$header .= "Subject: =?UTF-8?B?".base64_encode($Tema)."?=\r\n";
$header .= "To: ".$To."\r\n";
$header .="List-Unsubscribe: ".UNSUBSCRIBE_SF;
$body = "This is a MIME encoded message.";
$body .= "\r\n\r\n--" . $uid . "\r\n";
$body .= "Content-type: text/plain;charset=utf-8\r\n\r\n";
$body .= "Content-Transfer-Encoding: 8bit\r\n";
$body .= $text_us;
$body .= "\r\n\r\n--" . $uid . "\r\n";
$body .= "Content-type: text/html;charset=utf-8\r\n\r\n";
$body .= "Content-Transfer-Encoding: 8bit\r\n";