php代码：电子邮件不发送

my problem is i have a code to send email to users when click in some button its works for me in localhost but not working when i push it in heroku server

<?php

if (isset($_POST['submitcmt']) && $_POST['token'] == $dcs_user_info['token']) {
    //get rows in watchlist table if contest_id (job_id)==contest_id in database $watchlist_table = mysqli_query($conn, "SELECT * FROM watchlist WHERE contest_id='$contest_id'") or die("Error: " . mysqli_error($watchlist_table));
}

if (mysqli_num_rows($watchlist_table) > 0) {

    //get rows from content table
    $contentsTable = mysqli_query($conn, "SELECT * FROM contests WHERE id='$contest_id'")
    or die("Error: " . mysqli_error($conn));

    //to can use the rows in users table 
    $row_contents = mysqli_fetch_array($contentsTable, MYSQLI_ASSOC);

    //get rows from users table
    $userTable = mysqli_query($conn, "SELECT * FROM users WHERE id!='" . $dcs_user->user['id'] . "'")
        or die("Error: " . mysqli_error($conn));

    require 'vendor/vendor/phpmailer/phpmailer/PHPMailerAutoload.php';
    require 'vendor/autoload.php';

    $mail = new PHPMailer;
    $mail->IsSMTP();
    $mail->SMTPAuth = true;
    //$mail->SMTPDebug=2;
    //$mail->Debugoutput = 'html';

    $mail->SMTPOptions =
        [
            'ssl' => [
                'verify_peer' => false,
                'verify_peer_name' => false,
                'allow_self_signed' => true,
            ],
        ];

    $mail->Host = 'smtp.gmail.com';
    $mail->Username = 'faresalkhwaja@gmail.com';
    $mail->Password = 'elkhawajah1';
    $mail->SMTPSecure = 'ssl';
    $mail->Port = 465;

    //to select all the email in database
    while ($row_user = mysqli_fetch_array($userTable, MYSQLI_ASSOC)) {

        $mail->From = 'tasqat';
        $mail->FromName = 'tasqat';
        $mail->addReplyTo('faresalkhwaja@gmail.com', 'tasqat');
        $mail->addAddress($row_user['email'], $row_user['email']);

        $mail->Subject = "new comment";

        $actual_link = "http://$_SERVER[HTTP_HOST]$_SERVER[REQUEST_URI]";

        $mail->Body = "job title :" . $row_contents['title'] . "<br>" . "user name :" . $row_comment['display_name'] . "<br>" . "date : " . $row_comment['date'] . "<br>" . "comment : " . $row_comment['comment'] . "<br>" . "host : " . $actual_link;

        $mail->AltBody = 'this is body';
    }
}//end while loop

Your hosting provider likely has disabled emailing to prevent its system being used for spamming. You should contact them to see if they will enable it for you.

If they will not, you may want to consider using a third-party service.