Depending on how Mailster handles WP’s email processing (which is actually out-of-scope here), you could probably do it by adding a BCC when phpMailer is initialized. In a pure WP sense, that would add your BCC to any email being handled through wp_mail()
.
add_action( 'phpmailer_init', function( $phpmailer ) {
$phpmailer->addBCC( '[email protected]' );
});