PHP's mail() function doesn't work properly

I am using PHP's mail() function to send automated e-mails from my website and while testing, I noticed that when I sent mail from website by Gmail, then e-mails sent by php mailer are generating the following warning on the recipients end: This message may not have been sent by: example@gmail.com Learn more Report phishing. But when I use other emails (like yahoo, outlook), then I got no emails in my $contact_email. Please help me to solve this problem.

** I don't want to use Google app. and my domain email address.
** I just want If someone contact (fill the form ), then I just got email in my website.
Here the code:

global $_REQUEST;
$response = array('error'=>'');


    $user_name = substr($_REQUEST['user_name'], 0, 20);
    $user_email = substr($_REQUEST['user_email'], 0, 40);
    $user_msg = $_REQUEST['user_msg'];

    $contact_email = 'contact.arefin@gmail.com';    

    if (trim($contact_email)!='') {
        $subj = 'Message from Official Website';
        $msg = "Name: $user_name

        E-mail: $user_email
        Message: $user_msg";

        $head = "Content-Type: text/plain; charset=\"utf-8\"\n"
            . "X-Mailer: PHP/" . phpversion() . "\n"
            . "Reply-To: $user_email\n"
            . "To: $contact_email\n"
            . "From: $user_email\n";

        if (!@mail($contact_email, $subj, $msg, $head)) {

            $response['error'] = 'Error send message!';
        }
    } else 
            $response['error'] = 'Error send message!';

    echo json_encode($response);
    die();

?>