The “Sent by” happens when the ID of the sender does not match the owner of the mail file being used. For example, a secretary sends a note from his/her boss’s mail file. The recipient will see the message as From the boss, but Sent By the secretary. Another common scenario is when sending mail from a mail-in database. As to how to get rid of the label and it’s information: send the message using the ID of the database owner.
This can be caused when the mail file Owner’s name has changed. Select Tools — preferences, then replace the Mail file owner name from the Domino Directory.