From the RFC: http://www.xmpp.org/rfcs/rfc3921.html#rules

11.1.3: Server Rules for Handling XML Stanzas - Inbound Stanzas
Else if the JID is of the form <user@domain/resource> and no available resource matches the full JID, the recipient's server (a) SHOULD silently ignore the stanza (i.e., neither deliver it nor return an error) if it is a presence stanza, (b) MUST return a <service-unavailable/> stanza error to the sender if it is an IQ stanza, and (c) SHOULD treat the stanza as if it were addressed to <user@domain> if it is a message stanza.