An updated internet-draft is available from the IETF. This draft is a work item of the Enhancements to Internet email to support diverse service environments Working Group of the IETF.
Abstract: «The CATENATE extension to the Internet Message Access Protocol (IMAP) allows clients to create messages on the IMAP server which may contain a combination of new data along with parts of (or entire) messages already on the server. Using this extension, the client can catenate parts of an already existing message on to a new message without having to first download the data and then upload it back to the server.»