1. System, comprising: a central server, responsive to an invitation message from an inviting user to exchange content with an invited user, for providing a presence query; and a presence server, responsive to said presence query, for providing presence information relating to a registered user, wherein said central server is responsive to said presence information relating to said invited user registered at said presence server, for use in deciding whether said content is sent to said invited user, stored or refused, wherein said presence query and invitation message are communicated according to an application layer control protocol and wherein said presence information relating to said registered user pertains to a spatial location of said registered user; and wherein said central server is also responsive to said invitation message for providing a subscription query and wherein said system further comprises a messaging server, responsive to said subscription query, for providing notification information relating to a request from a subscribed user for notification of an event, and wherein said central server is responsive to said notification information for said subscribed user in deciding said content should be sent to said invited user, stored or refused.