You mean you only set the smtp envelope sender and not any From:, Sender: or Reply-To: headers in the mail itself. RT reads the information from the headers and not from smtp envelope (even though that usually will work if your e-mail runs through an MTA which adds the smtp envelope from in the Sender: header)
If you send e-mails without From: header it’s not really any useful test for normal clients. All e-mail clients I know automatically add a From: header.