Send all emails on the site as bcc. To make the snippet work, the subject field of the email should contain <bcc:emails>

send_emails_bcc.php

<?php

add_filter( 'wp_mail', function( $args ) {
    
    if ( false === strpos( $args['subject'], '<bcc:emails>' ) ) {
        return $args;
    }
    
    $email = $args['to'];
    
    $email = implode( ', ', $email );
    
    $args['subject'] = str_replace( '<bcc:emails>', '', $args['subject'] );
    
    $bcc = $email;
    
    $args['to'] = array( '[email protected]' );
    
    $args['headers'] .= "\r\nBcc: " . $bcc;
    
    return $args;
    
} );