If all of the available resources have a negative presence priority then the server SHOULD
either (a) store the message offline for later delivery
or (b) return a stanza error to the sender, which SHOULD be .
So client should nutify user that service has no offline storage and message can't be delivered
RFC6121: 8.5.2.1.1
So client should nutify user that service has no offline storage and message can't be delivered