HOW ENABLE SMTP?

Dear all,

This script was working fine sometimes ago, but now it does not works, there is an error message:

Warning: mail(): SMTP server response: 554 5.5.4 Your webserver has sent too many non-authenticated messages. Please enable SMTP Authentication in your script/code to continue sending messages.

How I enable SMTP in this script?

<?php

if (isset($_POST['g-recaptcha-response'])) {

    $captcha_data = $_POST['g-recaptcha-response'];

}

// Se nenhum valor foi recebido, o usuário não realizou o captcha

if (!$captcha_data) {

    echo "Por favor, confirme o captcha.";

    exit;

}
$resposta = file_get_contents("https://www.google.com/recaptcha/api/siteverify?secret=6LduXAcUAAAAAAD9VXEHrm-lL_Pg-YiWcCo2_bIS&response=".$captcha_data."&remoteip=".$_SERVER['REMOTE_ADDR']);
?>

<?php

//if ($resposta.success) {

//    echo "Obrigado por deixar sua mensagem!";

//} else {

//    echo "Usuário mal-intencionado detectado. A mensagem não foi enviada.";

//    exit;

//}

?>

<?php

if(isset($_POST['submit'])){


    $to = "contactme@quarup.com"; // this is your Email address

    $from = $_POST['xmail']; // this is the sender's Email address

    //$from = $_POST['contactme@quarup.com']; // this is the sender's Email address

    $first_name = $_POST['first_name'];

    $last_name = $_POST['last_name'];

      $xmail = $_POST['xmail'];

    $subject = "Form submission";

    $subject2 = "Copy of your form submission";

    $message = $first_name . " " . $last_name ."" .$xmail ." wrote the following:" . "\n\n" . $_POST['message'];

    $message2 = "Here is a copy of your message " . $first_name . "\n\n" . $_POST['message'];

    $headers = "From:" . $from;

    $headers2 = "From:" . $to;

      ini_set("sendmail_from", "contactme@quarup.com");
            

    mail($to,$subject,$message,$xmail, $headers);

  //  mail($from,$subject2,$message2,$xmail,$headers2); // sends a copy of the message to the sender

    echo "Mail Sent. Thank you " . $first_name . ", we will contact you shortly.";

    // You can also use header('Location: thank_you.php'); to redirect to another page.

    }



      ?>

Thank you in advance