Niente, ho provato come segue, ma non va...

Codice PHP:
<?php

    include_once('../../phpclass/class.phpmailer.php');

    if(isset($_POST['nomemitt'])&&isset($_POST['mailmitt'])&&isset($_POST['messmitt'])){
        
        $mail = new PHPMailer();
        $mail->CharSet = 'UTF-8';
        
        $mail->From = $_POST['mailmitt'];
        $mail->FromName = $_POST['nomemitt'];
        $mail->Subject = "Mail service";
        $html_body = '
        <html>
            <body>
                Messaggio:

                '.$_POST['messmitt'].'
            </body>
        </html>
        ';
        
        $mail->MsgHTML($html_body);
        $mail->AddAddress($_POST['mailaddress']);
        $mail->Send();
    
        echo $_POST['mailmitt']."
";
        echo $_POST['nomemitt']."
";
        echo $_POST['messmitt']."
";
        echo $_POST['mailaddress'];
        
        echo "ok";
    }
    else
    {
        echo "
        <div id=\"contactFormUsr\">
            <form method=\"POST\">
                <div class=\"cfUsrBox\">
                    <div class=\"cfUsrTitle\">
                        Messaggio per
                    </div>
                    <div class=\"nnuuaUsrForm\">"
                        .$u_art['nnuuadarte'].
                    "</div>
                </div>
                <div class=\"cfUsrBox\">
                    <div class=\"cfUsrTitle\">
                        Il tuo nome
                    </div>
                    <input type=\"text\" name=\"nomemitt\" class=\"lineCFUsr\" />
                </div>
                <div class=\"cfUsrBox\">
                    <div class=\"cfUsrTitle\">
                        La tua e-mail
                    </div>
                    <input type=\"text\" name=\"mailmitt\" class=\"lineCFUsr\" />
                </div>
                <div class=\"cfUsrBox\">
                    <div class=\"cfUsrTitle\">
                        Messaggio
                    </div>
                    <textarea name=\"messmitt\" class=\"txtaCFUsr\"></textarea>
                </div>
                <input type=\"hidden\" name=\"mailaddress\" value=\"".$u_usr['email']."\" />
                <input type=\"hidden\" name=\"username\" value=\"".$usr."\" />
                <div id=\"submitFormUsr\">
                    <input type=\"submit\" name=\"invia\" value=\"Invia\">
                </div>
            </form>
        </div>
        ";
    }
?>
Il risultato è che non viene inviata l'email e la popup sparisce senza che apparentemente accada nulla.
Dove sbaglio?