Cause

Someone sent mail without a valid recipient in the To: field. Thus, sendmail(1M) could not deliver the mail message. Using mail(1), the recipient's address might have been specified using spaces or non-alphanumeric characters. The mailtool(1) and mailx(1) commands try to prevent such problems by issuing Please specify a recipient or No recipients specified messages instead. If at least one valid recipient exists, each invalid recipient address will generate a User unknown message.