PHP邮件显示电子邮件中的BCC地址

My application sends emails to an Admin and to Customers upon payment is received. I would like to add a BCC field for a backup email and do not want it to show. I tried adding it as a BCC field but it is still showing when the email is received. The header reads as follows:

to me, bcc: the BCC e-mail, bcc: second BCC email

My code is as follows:

$to="email@hotmail.com";
$subject= "New order";
$headers = "MIME-Version: 1.0" . "
";
$headers .= "Content-type:text/html;charset=UTF-8" . "
";
$headers .= 'From: <email@gmail.com>' . "
";
$headers .= "CC:".PHP_EOL;
$headers .= "BCC: email@hotmail.com, email2@hotmail.com".PHP_EOL;

mail($to, $subject, $msg2, $headers);
mail($email, 'Payment Confirmation', $msg2customer, $headers);

I tried removing the CC field, still the same result.

*Note: When the emails are received by a @hotmail.com account the BCC are hidden, but when received by a @Gmail.com account the BCC are shown. I'm on local host, did not try on a live server. Thank you