Return the args(email) as it might have been customised by another extension (eg. wp-mail-html) and we don't want to drop those customisations.
Client is using https://wordpress.org/plugins/wp-html-mail/ which allows for pretty/html WordPress mails. It uses
wp_mail filter with priority 12 so is run before wp-civicrm-mailer which uses
wp_mail filter with priority 100. But you need to pass through the arguments otherwise it defaults to before the filter was called and you get no customisation.