too long

I receive email correctly. For safety purpose I've hidden my recaptcha secret key but this part is working fine. When I receive the email the subject's special characters are displayed correctly. However, if french accents are sent in the message (éèàç, for example) the are displayed like this (éèà ç). As this is a french website, this is a real problem. I tried utf8_encode($message) in the mail function but to no avail. I looked at other stackoverflow posts but none helped. Also in the message this part is displayed as Content-type: text/plain; charset=UTF-8'. Can this be avoided?

<?php
    if(isset($_POST['valider'])) {
            $url = 'https://www.google.com/recaptcha/api/siteverify';
            $privateKey = "secret";

            $response = file_get_contents($url."?secret=".$privateKey."&response=".$_POST['g-recaptcha-response']."&remoteip=".$_SERVER['REMOTE_ADDR']);
            $data = json_decode($response);
            if(isset($data->success) AND $data->success==true){

                    $rusty = 'email@email.com';
                    $destinataire = $_POST["email"];
                    $message = utf8_encode(wordwrap($_POST['message']));
                    $subject = "Question pour la méthode EtVoilà";
                    $headers = 'From:' .  $destinataire."
";
                    $header_ = 'MIME-Version: 1.0' . "
" . 'Content-type: text/plain; charset=UTF-8' . "
";
                    if(mail($rusty,'=?UTF-8?B?'.base64_encode($subject).'?=',$message,$header_ . $headers)) {
                    header('Location: index.php?MessagePass=True');
            } else {
                    header('Location: index.php?MessageError=True');
                    }
            }
    }
?>

I found that the function iconv does the trick for me, here's the new part in case someone has the same problem. I just add the second line in the code below.

                    $message = wordwrap($_POST['message']);
                    $message = iconv("UTF-8", "ISO-8859-1//TRANSLIT", $message);