PHP邮件BCC多个收件人没有发送到BCC

I am trying to send a BCC ( Blind Carbon Copy ) of an email.

Here is my class:

class email{
    function __construct(){
    }
    public function send($to, $from, $subject, $message){
        $header = $this->nl('MIME-Version: 1.0') .
        $this->nl('Content-Type: text/plain; charset=utf-8') .
        $this->nl('X-Priority: 1') .
        $this->nl('Importance: High') .
        $this->nl('X-MSMail-Priority: High') . 
        $this->nl('Bcc: email1@email.com,email2@email.com') .
        $this->nl('X-Mailer: PHP/' . phpversion());
        return mail(
            $to,
            $subject,
            $message,
            $header,
            '-f ' . $from
        );
    }
    private static function nl($str){
        return $str . "
";
    }
}

When sending the email the recipient receives the email, but the BCC recipients do not.

Solution thanks to Dave:

class email{
    function __construct(){

    }
    public function send($to, $from, $subject, $message){
        $header = $this->nl('MIME-Version: 1.0') .
            $this->nl('Content-Type: text/plain; charset=utf-8') .
            $this->nl('X-Priority: 1') .
            $this->nl('Importance: High') .
            $this->nl('X-MSMail-Priority: High') . 
            $this->nl('BCC: rick <rick@email.com>; angela <angela@email.com>') .
            $this->nl('X-Mailer: PHP/' . phpversion());
        return mail(
            $to,
            $subject,
            $message,
            $header,
            '-f ' . $from
        );
    }
    private function nl($str){
        return $str . "
";
    }
}

BCC is case sensitive in email headers I think so it should be BCC: not Bcc: also think bcc email addresses needs to be separated by ;

Though personally I'd also recommend swapping to a proper mail component such as swiftmailer gives you far better error reporting/trapping and functionality over and above the built in php mail() function