A MUC service might rewrite the id attribute, see
http://xmpp.org/extensions/xep-0045.html#message
"Note well that for tracking purposes this service assigns a new 'id' to
each message it generates (here using a UUID as defined in RFC 4122)"
I bet some services do this... making the id at the recipient unpredictable.
We should probably fix MUC on this - I hadn't noticed in the examples
what was going on and read the text to mean "The id sent to the room
may not be the id you specified" not "Everyone in the room may get a
different id" - which will break things.
As you pointed out, message receipts is likely affected by this, too,
and http://xmpp.org/extensions/xep-0184.html#when-groupchat does not seem
to expect id changes either. +1 on fixing MUC, this is somewhat unexpected
and I doubt that per-participant is really needed for tracking.
