// RFC 6121 8.5.1. No Such User http://xmpp.org/rfcs/rfc6121.html#rules-localpart-nosuchuser
// If the 'to' address specifies a bare JID <localpart@domainpart> or full JID <localpart@domainpart/resourcepart> where the domainpart of the JID matches a configured domain that is serviced by the server itself, the server MUST proceed as follows.
// If the user account identified by the 'to' attribute does not exist, how the stanza is processed depends on the stanza type.