$mail = new MailMessage();
$mail->setBcc([
    'bcc_recipient_one@example.com',
    'bcc_recipient_two@example.com',
    'bcc_recipient_three@example.com',
]);

$mail = new MailMessage();
$mail->setFrom('test@stackoverflow.com', 'Stackoverflow Tester');
$mail->addTo('email@stackoverflow.com', 'Stackoverflow  Recipient');
$mail->setSubject('Stackoverflow Test');
$mail->setBcc(['bcc_one@so.com', 'bcc_two@so.com', 'bcc_three@so.com']);

Date: Wed, 18 Jan 2018 19:11:36 +0100
From: Stackoverflow Tester <test@stackoverflow.org>
To: email@stackoverflow.com
Subject: Stackoverflow Test
Bcc: bcc_one@so.com,
 bcc_two@so.com,
 bcc_three@so.com

public function getFieldValue($format = HeaderInterface::FORMAT_RAW)
{
    $value = parent::getFieldValue($format);
    return str_replace(Headers::FOLDING, ' ', $value);
}

$bcc = new MyModuleHeaderBcc();
$bcc->getAddressList()->addMany([
    'bcc_one@so.com',
    'bcc_two@so.com',
    'bcc_three@so.com',
]);
$mail->getHeaders()->addHeader($bcc);

Date: Wed, 18 Jan 2018 19:11:36 +0100
From: Stackoverflow Tester <test@stackoverflow.org>
To: email@stackoverflow.com
Subject: Stackoverflow Test
Bcc: bcc_one@so.com, bcc_two@so.com, bcc_three@so.com