Running prosody 0.10 with mod_carbons, if you send a message to your own bare JID, it will be carbon-copied to your other client.
The carbon copy is lacking the 'to' element (it is stripped from the original message by prosody):
Original message as sent by /poezio:
<message to="firstname.lastname@example.org" id="X" type="chat">
Carbon-copy as received by /yaxim:
<message to="email@example.com/yaxim" from="firstname.lastname@example.org" type="chat">
<message id="X" from="email@example.com/poezio" type="chat">
According to https://xmpp.org/rfcs/rfc6120.html#stanzas-attributes-to-c2s this implies that the carbon-copied message was intended to be processed by the server (this is different from being addressed to your bare JID, thanks RFC6121).
Either way, it is a bad idea to have carbon-copied messages lack either 'from' or 'to'.