PHP E-Mail Encoding?

As far as I know PHP does not support UTF-8 as default encoding for its strings. You need to use the relevant encoding/handling functions for the encoding you would prefer.

Also add a Content-Type:text/html;charset=utf-8 to your email headers so the email clients will display the characters correctly (or replace with your encoding of choice).

You didn’t specify the type and encoding of your content. Try this:

$headerFields = array(
    "From: {$visitor_email}",
    "MIME-Version: 1.0",
    "Content-Type: text/html;charset=utf-8"
);
$mail_sent = mail($client_email, $title, $message, implode("\r\n", $headerFields));

Use this code

function mail_send($arr)
{
    if (!isset($arr['to_email'], $arr['from_email'], $arr['subject'], $arr['message'])) {
        throw new HelperException('mail(); not all parameters provided.');
    }

    $to      = empty($arr['to_name']) ? $arr['to_email'] : '"' . mb_encode_mimeheader($arr['to_name']) . '" <' . $arr['to_email'] . '>';
    $from    = empty($arr['from_name']) ? $arr['from_email'] : '"' . mb_encode_mimeheader($arr['from_name']) . '" <' . $arr['from_email'] . '>';

    $headers = array
    (
        'MIME-Version: 1.0',
        'Content-Type: text/html; charset="UTF-8";',
        'Content-Transfer-Encoding: 7bit',
        'Date: ' . date('r', $_SERVER['REQUEST_TIME']),
        'Message-ID: <' . $_SERVER['REQUEST_TIME'] . md5($_SERVER['REQUEST_TIME']) . '@' . $_SERVER['SERVER_NAME'] . '>',
        'From: ' . $from,
        'Reply-To: ' . $from,
        'Return-Path: ' . $from,
        'X-Mailer: PHP v' . phpversion(),
        'X-Originating-IP: ' . $_SERVER['SERVER_ADDR'],
    );

    mail($to, '=?UTF-8?B?' . base64_encode($arr['subject']) . '?=', $arr['message'], implode("\n", $headers));
}

from:http://php.net/manual/en/function.mail.php