Copied from the XMPP spec:
11. Server Rules for Handling XML Stanzas
For presence stanzas other than those of type "probe", the server MUST deliver the stanza to all available resources; for presence probes, the server SHOULD reply based on the rules defined in Presence ProbesPresence Probes. In addition, the server MUST NOT rewrite the 'to' attribute (i.e., it MUST leave it as <user@domain> rather than change it to <user@domain/resource>).