A domain may also define backup servers; they have no mailboxes and forward messages without changing any part of their envelopes.
[3] By contrast, primary servers can deliver a message to a user's mailbox and/or forward it by changing some envelope addresses.
This field holds the address to which mail-systems must send bounce messages — reporting delivery-failure (or success) — if any.
By contrast, the terms remailing or redistribution can sometimes mean re-sending the message and also rewriting the "envelope sender" field.
[7] Intra domain redirection complies with SPF as long as the relevant servers share a consistent configuration.
Note that including all the headers discloses much information about the message, such as the servers that transmitted it and any client-tag added on the mailbox.
RFC 821, Simple Mail Transfer Protocol, by Jonathan B. Postel in 1982, provided for a forward-path for each recipient, in the form of, for example, @USC-ISIE.ARPA, @USC-ISIF.ARPA: Q-Smith@ISI-VAXA.ARPA — an optional list of hosts and a required destination-mailbox.
Otherwise, in the case of insufficient destination-information but where the server knew the correct destination, it could take the responsibility to deliver the message by responding as follows: The concept at that time envisaged the elements of the forward-path (source route) moving to the return-path (envelope sender) as a message got relayed from one SMTP server to another.
Even if the system discouraged the use of source-routing,[9] dynamically building the return-path implied that the "envelope sender" information could not remain in its original form during forwarding.
In 2008, RFC 5321 still mentions that "systems may remove the return path and rebuild [it] as needed", taking into consideration that not doing so might inadvertently disclose sensitive information.
The reference SMTP implementation in the early 1980s was sendmail, which provided for ~/.forward files, which can store the target email-addresses for given users.
[12] One can configure some email-program filters to automatically perform forwarding or replying actions immediately after receiving.
Formerly only trusted users could utilize the command-line switch for setting the envelope sender, -f arg; some systems disabled this feature for security reasons.