Splits wp_mail() with more than 40 bcc into multiple batches

postmark-split-wp-mail.php

<?php
/**
 * Splits a wp_mail() call with more than 40 bcc
 * headers into multiple batches.
 *
 * Postmark only accepts 50 bcc, so this ensures the API call doesn't fail.
 *
 * @param array $args Original arguments passed to wp_mail().
 * @return
 */
add_filter( 'wp_mail', function( $args ) {
	// Scenarios we can't process.
	if ( empty( $args['headers'] )
		|| ! is_array( $args['headers'] )
		|| count( $args['headers'] ) < 40 ) {
		return $args;
	}

	// Capture all of the existing bcc headers.
	$mod_headers = $args['headers'];
	$bcc_headers = [];
	foreach ( $mod_headers as $i => $header ) {
		if ( 0 === stripos( $header, 'bcc:' ) ) {
			unset( $mod_headers[ $i ] );
			$bcc_headers[] = $header;
		}
	}

	// Don't need to modify if there are less than 40 recipients.
	if ( count( $bcc_headers ) < 40 ) {
		return $args;
	}

	// Break the bcc emails into chunks of less than 40.
	$mod_headers = array_values( $mod_headers );
	foreach ( array_chunk( $bcc_headers, 40 ) as $i => $chunk ) {
		// Keep the first chunk for the original send.
		if ( 0 === $i ) {
			$first_chunk = $chunk;
			continue;
		}
		// Send out the chunk.
		$chunk_headers = array_merge( $mod_headers, $chunk );
		wp_mail(
			$args['to'],
			$args['subject'],
			$args['message'],
			$chunk_headers,
			$args['attachments']
		);
	}
	// Use the $first_chunk for the original send.
	$args['headers'] = array_merge( $mod_headers, $first_chunk );

	return $args;
});