Chops up WordPress out-going emails so that number of bcc addressees stays below a configured limit

Ovni_chop_up_emails

<?php
/*
Plugin Name: Ovni chop up emails
Description: Prevents number of Bcc addresees exceeding defined limit by breaking it up into multiple emails.
Author: Tony Voss
	Version:	1.0		1 Mar 2019		

	History
	Version:	1.0		1 Mar 2019		Initial version

	Limitations:
			(1) If an email contains mutiple cc addressees it notes it in the error log but lets the email through.  Not seen any such emails from my websites.
*/

// Exit if accessed directly
defined( 'ABSPATH' ) || exit;

function ovni_chop_email($args){
	/* configuration */
	$max_bcc = 10;	// maximum number of Bcc addresses allowed
	$debug = false;	// whether want debug
	if ($debug){
		ob_start();
		var_dump($args);
		$result = ob_get_clean();
		error_log("Entering ovni_chop_email\n" . $result);
		}
	$headers = $args['headers'];
	if (empty($headers)) return($args); // if no headers, nothing to do
	// pick up CC/BCC, collating them from headers
	if (isset($args['headers'])) {
		$cc = array();
		$bcc = array();
		$other = array();
		if (!is_array($headers)) {
			$headers = explode("\n", str_replace("\r\n", "\n", $headers));
		}
		foreach ($headers as $header) {
			if ($header) {
				switch (strtolower(substr($header, 0, 3))) {
					case 'cc:':
						$cc[] = $header;
						error_log("Found cc:" . $header);
						break;
					case 'bcc':
						$bcc[] = $header;
						break;
					default:
						$others[] = $header;
						break;
					}
				}
			if (!empty($cc)){
				// not yet supporting cc
				error_log("Ovni_chop_mail saw addressee in cc field for email: " . $args['subject']);
				return($args);
				}
			}
		
		if (sizeof($bcc) == 1){ // may be a list of adressees with a single bcc header, as from Subscribe2
			$bcc = explode("\n", str_replace(",", "\nBcc:", $bcc[0]));
			}
		if (sizeof($bcc) <= $max_bcc)	return($args);  // This one can go straight through - nothing to do
		}
	// we need to chop up this email into multiples
	remove_filter('wp_mail', 'ovni_chop_email',5); // We do not want to be called back each time we send one of the parts 
	while (sizeof($bcc) > $max_bcc){
		if($debug) error_log("Chopping up - bcc=". strval(sizeof($bcc)));
		$this_batch = array_slice($bcc, 0, $max_bcc);
		$bcc = array_slice($bcc, $max_bcc);  // reduce to those not in this batch
		$headers = array_merge($others, $this_batch);
		if (!wp_mail($args["to"], $args["subject"],  $args["message"], $headers, $args["attachments"])) error_log("Ovni_chop_email returned error");
		}
	// We are here because we have done all but the last chunk.  Return and let original call do this one.
	$headers = array_merge($others, $bcc);
	$args['headers'] = $headers;
	add_filter('wp_mail', 'ovni_chop_email',5);  // restore the filter
	return($args);
	}
add_filter('wp_mail', 'ovni_chop_email',5);