The Sieve scripting language allows users to control handling and
disposal of their incoming e-mail. By default, an e-mail message
that is processed by a Sieve script is saved in the owner's "inbox".
Actions such as "fileinto" and "redirect" cancel this default
behavior.
This document defines a new keyword parameter, ":copy", to be used
with the Sieve "fileinto" and "redirect" actions. Adding ":copy" to
an action suppresses cancellation of the default "inbox" save. It
allows users to add commands to an existing script without changing
the meaning of the rest of the script.
The Abstract:
http://tools.ietf.org/html/rfc3894