Connection failed. Error #2: stream_socket_enable_crypto(): Peer certificate with PHPMailer

I'm using below code for sending mail. It's working fine in gmail server, but it's not working for my domain.
It's showing an error like

Connection failed. Error #2: stream_socket_enable_crypto(): Peer certificate CN=`*.999servers.com'

How can I solve this issue?
This is my code so far:

require("PHPMailer/src/PHPMailer.php");
require("PHPMailer/src/SMTP.php");
$mail = new PHPMailer\PHPMailer\PHPMailer();
$mail = new PHPMailer\PHPMailer\PHPMailer(true);
$mail->IsSMTP();
$mail->SMTPDebug = 4;
$mail->SMTPAuth = true;
$mail->SMTPSecure = 'tls';
$mail->Host = "mail.mydomain.in";
$mail->Port = 587;
$mail->IsHTML(true);
$mail->Username = "mail";
$mail->Password = "password";

$mail->SetFrom($admin_user_mail);
$mail->AddAddress($login_user_mail);
$mail->AddCC($admin_user_mail);
$mail->Subject = "Mail Subject";
$mail->Body = "Mail Content";


if($mail->send()) 
{   
    echo "Mail Send";
} else 
{
    echo "Mail Not sent";
}