The following facts guarantee that certain cross-host relay behavior is safe. Also, a host that does not accept mail from the network can relay messages to a nearby hub. A relay adds a new Received header field to the top of the output. Other than this, the output header, body, and envelope are exactly the same as the input header, body, and envelope. Bouncers A bouncer is a pre-delivery automailer that lets the envelope sender know what happened to a message.
Most bouncers send failure notices.
Some bouncers, such as vacation servers and echo servers, send success notices. Unlike a relay, a bouncer produces output with a new header, not simply a copy of the input header. The body of a failure notice includes a copy of the entire input message. Such failures are generally caused by poorly configured hosts that produce normal messages with faulty envelope sender addresses. The output envelope recipient is predetermined.