sendmail.inc.original.php 
+++ sendmail.inc.mime.php 
@@ -33,23 +33,23 @@ 

$email_subject = '=?'.$charset.'?B?'.base64_encode(str_replace("\r", '', str_replace("\n", '', '['.$bbname.'] '.$email_subject))).'?='; 
$email_message = chunk_split(base64_encode(str_replace("\r\n.", " \r\n..", str_replace("\n", "\r\n", str_replace("\r", "\n", str_replace("\r\n", "\n", str_replace("\n\r", "\r", $email_message))))))); 

$email_from = $email_from == '' ? '=?'.$charset.'?B?'.base64_encode($bbname)."?= <$adminemail>" : (preg_match('/^(.+?) \<(.+?)\>$/',$email_from, $from) ? '=?'.$charset.'?B?'.base64_encode($from[1])."?= <$from[2]>" : $email_from); 

foreach(explode(',', $email_to) as $touser) { 
$tousers[] = preg_match('/^(.+?) \<(.+?)\>$/',$touser, $to) ? ($mailusername ? '=?'.$charset.'?B?'.base64_encode($to[1])."?= <$to[2]>" : $to[2]) : $touser; 
} 
$email_to = implode(',', $tousers); 

-$headers = "From: $email_from{$maildelimiter}X-Priority: 3{$maildelimiter}X-Mailer: Discuz! $version{$maildelimiter}MIME-Version: 1.0{$maildelimiter}Content-type: text/plain; charset=$charset{$maildelimiter}Content-Transfer-Encoding: base64{$maildelimiter}"; 
+$headers = "From: $email_from{$maildelimiter}X-Priority: 3{$maildelimiter}X-Mailer: Discuz! $version{$maildelimiter}MIME-Version: 1.0{$maildelimiter}Content-type: ".(true === is_string($sendmail_mime) ? $sendmail_mime : 'text/plain')."; charset=$charset{$maildelimiter}Content-Transfer-Encoding: base64{$maildelimiter}"; 

$mail['port'] = $mail['port'] ? $mail['port'] : 25; 

if($mail['mailsend'] == 1 && function_exists('mail')) { 

@mail($email_to, $email_subject, $email_message, $headers); 

} elseif($mail['mailsend'] == 2) { 

if(!$fp = fsockopen($mail['server'], $mail['port'], $errno, $errstr, 30)) { 
$errorlog(' *** TP', "($mail[server]:$mail[port]) CONNECT - Unable to connect to the *** TP server", 0);