WP404: Send an email to Eric Mann every time there's a 404

functions.php

<?php

/**
 * Email the error log to Eric Mann every time a 404 error occurs.
 *
 * @param array $report The WP404 report.
 * @return array The (untouched) WP404 report.
 */
function mytheme_spam_eric_with_404s( $report ) {
  wp_mail(
    'ericmann@example.com',
    __( 'New 404 error', 'my-theme-text-domain' ),
    print_r( $report, true )
  );
  
  return $report;
}
add_filter( 'wp404_report_data', 'mytheme_spam_eric_with_404s', 999999999 );