All Internet e-mail is sent using a protocol called SMTP (Simple Mail Transfer Protocol). In SMTP, mail is sent in units called envelopes. First the address of the envelope sender is sent, followed by one or more envelope recipient addresses. Finally the actual message is sent, headers and body together. Note that the final recipient sees only the headers and body. Envelope information is generally visible only to sendmail and its Milters.

When a user replies to an e-mail message, the reply is generally sent to the header sender — the address listed in the From: header. But when a message is bounced, the bounce reply is...