현재 잘 사용하고 있는 내용입니다.
약간 늦을때가 있지만 html 적용되어서 잘 옵니다.

    // 메일발송 사용을 하지 않는다면
    if (!$config[cf_email_use]) return;

    $fname  = "=?$g4[charset]?B?" . base64_encode($fname) . "?=";
    $subject = "=?$g4[charset]?B?" . base64_encode($subject) . "?=";

    $header  = "Return-Path: <$fmail>\n";
    $header .= "From: $fname <$fmail>\n";
    $header .= "Reply-To: <$fmail>\n";
    if ($cc)  $header .= "Cc: $cc\n";
    if ($bcc) $header .= "Bcc: $bcc\n";
$header .= "MIME-Version: 1.0\n";
    $header .= "X-Mailer: SIR Mailer 0.92 (sir.co.kr) : $_SERVER[SERVER_ADDR] : $_SERVER[REMOTE_ADDR] : $g4[url] : $_SERVER[PHP_SELF] : $_SERVER[HTTP_REFERER] \n";

    if ($file != "") {
        $boundary = uniqid(time());

        $header .= "Content-type: MULTIPART/MIXED; BOUNDARY=\"$boundary\"\n\n";
        $header .= "--$boundary\n";
    }

$type=2;

if ($type) {
        $header .= "Content-Type: TEXT/HTML; charset=$g4[charset]\n";
        if ($type == 2)
            $content = nl2br($content);
    } else {
        $header .= "Content-Type: TEXT/PLAIN; charset=$g4[charset]\n";
        $content = stripslashes($content);
    }


    if ($file != "") {
        foreach ($file as $f) {
            $header .= "\n--$boundary\n";
            $header .= "Content-Type: APPLICATION/OCTET-STREAM; name=\"$f[name]\"\n";
            $header .= "Content-Transfer-Encoding: BASE64\n";
            $header .= "Content-Disposition: inline; filename=\"$f[name]\"\n";

            $header .= "\n";
            $header .= chunk_split(base64_encode($f[data]));
            $header .= "\n";
        }
        $header .= "--$boundary--\n";
    }
    @mail($to, $subject, $content, $header);
}

mailer.lib.php 파일 이메일로 보내드렸습니다.

감사합니다!!