Email forwarding

Email forwarding generically refers to the operation of re-sending an email message delivered to one email address on to a possibly different email address. The term forwarding has no specific technical meaning. Users and administrators of email systems use the same term when speaking of both server-based and client-based forwarding.

Email forwarding can also redirect mail going to one address and send it to one or several other addresses. Vice versa, email items going to several different addresses can converge via forwarding to end up in a single address in-box.

The domain name (the part appearing to the right of @ in an email address) defines the target server(s) for the corresponding class of addresses. A domain may also define backup servers; they have no mailboxes and forward messages without changing any part of their envelopes. By contrast, primary servers can deliver a message to a user's mailbox and/or forward it by changing some envelope addresses. ~/.forward files (see below) provide a typical example of server-based forwarding to different recipients.

Email administrators sometimes use the term redirection as a synonym for server-based email-forwarding to different recipients. Protocol engineers sometimes use the term Mediator to refer to a forwarding server.

Because of spam, it is becoming increasingly difficult to reliably forward mail across different domains, and some recommend avoiding it if at all possible.

Plain message-forwarding changes the envelope recipient(s) and leaves the envelope sender field untouched. The "envelope sender" field does not equate to the From header which Email client software usually displays: it represents a field used in the early stages of the SMTP protocol, and subsequently saved as the Return-Path header. This field holds the address to which mail-systems must send bounce messages — reporting delivery-failure (or success) — if any.

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