it is of type "normal" and contains a <body> element.
it is of type "error" and it was sent in response to a &MESSAGE; that was eligible for carbons delivery (Note that as this would require message tracking and correlation on the server, it is unlikely to be generally appropriate for most implementations).
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it matches the MUC-related rules outlined below.
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To properly handle messages exchanged with a MUC (or similar service), the server must be able to identify MUC-related messages. This can be accomplished by tracking the clients' presence in MUCs, or by checking for the <x xmlns="http://jabber.org/protocol/muc#user"> element in messages. The following rules are suggested for MUC-related messages:
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A &MESSAGE; of type "groupchat" SHOULD NOT be carbon-copied.
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A &MESSAGE; containing a &xep0249; SHOULD be carbon-copied.
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A &MESSAGE; containing a Mediated Invitation SHOULD be carbon-copied.
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A private &MESSAGE; from a local user to a MUC participant (sent to a full JID) SHOULD be carbon-copiedThe server SHOULD limit carbon-copying to the clients sharing a Multi-Session Nick in that MUC, and MAY inject the <x/> element into such carbon copies. Clients SHOULD ignore carbon-copies of MUC-PMs related to a MUC they are not joined to..
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A private &MESSAGE; from a MUC participant (received from a full JID) to a local user SHOULD NOT be carbon-copied (these messages are already replicated by the MUC service to all joined client instances).
As this is a implementation detail of servers, clients MUST NOT rely on the server implementing a particular set of rules for which messages are eligible for Carbons delivery.
Future specifications may have more precise requirements on which messages need to be eligible for carbons delivery; such future specifications will provide their own discovery and negotiation mechanisms, such that a client negotiating Carbons using the protocol defined in this specification will cause the server to consider messages eligible for Carbons delivery based on the requirements described herein.