php sendmail form fails, worked for years

Question

I have a php contact form which has been working reliably for several years up until Jan 2022. It uses a Google reCaptcha. Apparently it fails at the mailcheck part which is near the end and prints out the "MAIL Sending Failed" message

<?php
//dont need these
error_reporting(E_ALL);
ini_set('display_errors', 1);

// check form is submitted
if ( isset($_POST['form_submit']) ) {

    // get values
    $error = ''; //error variable
    $visitor_name    = $_POST['name'];
    $visitor_email   = $_POST['email'];
    $visitor_message = $_POST['message'];
    $captcha         = $_POST['g-recaptcha-response'];

    //required values
    if ( empty($visitor_name) || empty($visitor_email) ) {
        $error = "<h3>Name and Email are required. END.</h3>";
    }

    //required captcha
    if ( empty($captcha) ) {
        $error = "<h3>Please check the the captcha form. END.</h3>";
    }

    // if no errors
    if( empty($error) ){

        $secretKey = " *** hidden *** ";
        $ip = $_SERVER['REMOTE_ADDR'];
        // post request to server
        $url = 'https://www.google.com/recaptcha/api/siteverify?secret=' .
        urlencode($secretKey) .  '&response=' . urlencode($captcha);
        $response = file_get_contents($url);
        $responseKeys = json_decode($response, true);
        // should return JSON with success as true

        if ($responseKeys["success"]) {
            echo '<h3>Captcha Success. Writing Mail...</h3>';

         // write mail
         $to = " *** my user email ***.com";
         $email_subject = "C&H form v2c inquiry";
         // To send HTML mail, the Content-type header must be set
         // If the From header is not set, then Luxsci servers reject
         //    $headers = "";
         $headers = "From: *** my email ***.com \r\n";
         $headers .= "Reply-To: $visitor_email \r\n";
         $headers .= "MIME-Version: 1.0 \r\n";
         $headers .= "Content-type: text/html; charset=iso-8859-1 \r\n";
         $headers .= "X-Mailer: PHP/".phpversion();

        $email_body = "message from " . $visitor_name . "<br>" .
            "sender's email: " . $visitor_email . "<br>" .
            "here is the message: " . $visitor_message;
        // compose headers

        //Send the mail
        $mail_check = mail($to, $email_subject, $email_body, $headers);
        if ($mail_check) {
            echo "<h5>Mail sending successful. Redirecting... </h5>";
            echo '<script> window.location.href = "thank_you_CG.html"; </script>';
        } else {
            echo "<h5>MAIL sending failed.</h5><br<br>
                    <h3> This is the failure message it sends out HERE  </h3>";
        }

    } else { // if response not success
         echo '<h3>reCaptcha verification failed!</h3>';
         echo "Response from reCaptcha: <br>";
         print_r($responseKeys);
     }

     } else { //if errors
        echo $error;
        exit;
    }
 }

I can also add the html but there seems to be no problem with that. As mentioned this php form was working fine for years then stopped. I've heard that the Google recaptcha uses a smaller string now?