MessageSent: SHOULD include the -sender, -sender-id, -sent headers
MessageSent: SHOULD include the -sender, -sender-id, -sent headers

This is an instance of the general principle that the message should look
as close as possible to what other users will receive; in an XMPP MUC,
the message-sender(-id) should be channel-specific.

TelepathyIM/telepathy-spec,freedesktop-unofficial-mirror/telepathy__telepathy-spec