Cause

Somebody sent mail without a valid recipient in the To: field, so 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 this by issuing "Please specify a recipient" or "No recipients specified" messages instead. If there is at least one valid recipient, each invalid recipient address will generate a "User unknown" message.