Description of feature: When a user session is very long, typically multiple weeks or months, their contacts may have had their presence desynchronised, often due to remote server outages but there are other causes. This feature would have the server send a presence probe to every contact, to get missed presences, and remove stale resources by sending an unavailable presence to the user. It doesn’t need to be often, I think once a day would be enough, maybe using some randomised delay so that not all of them get the probe at the same time.