while you installed squirrelmail-decode-1.2

but still had the problem about "REPLY" charset error . (big5、gb2312...)

just edit " ＳRC/compose.php"

about... 930~950

            default:
                break;
        }
        session_write_close();
        sqimap_logout($imapConnection);
    }
    +// 3wa start
    +$send_to=mb_convert_encoding($send_to,$default_charset,'UTF-8');
    +$send_to_cc=mb_convert_encoding($send_to_cc,$default_charset,'UTF-8');
    +$send_to_bcc=mb_convert_encoding($send_to_bcc,$default_charset,'UTF-8');
    +$subject=mb_convert_encoding($subject,$default_charset,'UTF-8');
    +$mailprio=mb_convert_encoding($mailprio,$default_charset,'UTF-8');
    +$body=mb_convert_encoding($body,$default_charset,'UTF-8');
    +$identity=mb_convert_encoding($identity,$default_charset,'UTF-8');
    +// 3wa end
    $ret = array( 'send_to' => $send_to,
            'send_to_cc' => $send_to_cc,
            'send_to_bcc' => $send_to_bcc,
            'subject' => $subject,
            'mailprio' => $mailprio,
            'body' => $body,
            'identity' => $identity );

That's all, you win.