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.
Server-based forwarding
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.