The Friends System is based on an automatic responder. This responder sends a message to all unknown email addresses asking them to confirm they exist and are indeed a person and not another automatic email or spam robot.

If you have received a message containing a link to this page then the person to whom you were sending that email is using the Friends System.

Once you have replied to the message you will not see it again, unless the person removes you from their list of friends. So simply reply and you will be added to their friends list and can continue to send them email without being harassed by the Friends System again.