The "Envelope" protocol was never implemented. The closest thing to "envelope" is now the message headers proposal (XEP-0033). For historical purposes, the original proposal is provided below.
414 DeLong St.
This extension allows clients to express to each other identities and message recipients in a rich way. Full names or nicknames can be sent, message forwarding headers, requested reply-to addresses, etc. It is often used within server-side functionality to note when messages are forwarded. The server otherwise does not use this namespace for actual routing or delivery purposes, that is entirely client-side.
<x xmlns="jabber:x:envelope"> <to jid="user@server">John</to> <cc jid="email@example.com">Some guy</cc> <replyto jid="firstname.lastname@example.org"/> <from jid="email@example.com">The Thing</from> <forwardedby jid="admin@server">administrator</forwardedby> </x>