public function displayContent()
    {
        $this->processAddressFormat();

        // символы для e-mail
        $abc = array(a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z);
        // количество символов до @
        $num = 10;
        // домен для e-mail
        $domen = '@mydomain.ru';
        for ($i = 1; $i <= $num; $i++) {
          $id = array_rand($abc);
          $email .= $abc[$id];
        }
        $email = $email.$domen;
        self::$smarty->assign('rand_email', $email);

        parent::displayContent();
        self::$smarty->display(_PS_THEME_DIR_.'authentication.tpl');
    }

<input type="text" class="text" id="guest_email" name="guest_email" value="{if isset($smarty.post.guest_email)}{$smarty.post.guest_email}{/if}">                

<input type="text" class="text" id="guest_email" name="guest_email" value="{if isset($smarty.post.guest_email)}{$smarty.post.guest_email}{else}{$rand_email}{/if}">              

public function displayContent()
    {
        $this->processAddressFormat();

        // символы для e-mail
        $abc = array(a,b,c,d,e,f,g,h,i,j,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z);
        // количество символов до @
        $num = 3;
        // домен для e-mail
        $domen = '@mydomain.ru';
        for ($i = 1; $i <= $num; $i++) {
          $id = array_rand($abc);
          $email .= $abc[$id];
        }
        $email = $email.time().$domen;
        self::$smarty->assign('rand_email', $email);

        parent::displayContent();
        self::$smarty->display(_PS_THEME_DIR_.'authentication.tpl');
    }