mailx has a somewhat cryptic -r option but if you have control over the script, use something else than mailx if it doesn't suit your needs. mailx doesn't really have any significant intelligence of its own, it just passes stuff to sendmail AFAIK, so you might as well invoke sendmail directly -- though then you have to be more careful to make sure the mail message is valid and has all the necessary headers. That would be To:, From:, and Subject:, basically.