When using the "Send Email" function on mb.org, checking "Send a copy to my own email address" causes a Bcc header to be added with the sending user's email address. This header is ignored by the transmitting MUA, meaning that the sending user doesn't get a copy of the mail, and more seriously, the receiving user receives a message with a Bcc header containing the sending user's email address, irrespective of whether they've permitted it to be revealed.