[JDEV] Getting a user's presence state

[Ramy M. Hassan]

I am writing a system wide email notification system. It should send a 
notification message when an email arrives to a user only when he is online. 
This means that I have to know the presence state of the user before I decide 
to send him anything.  Is there any good way to know this information 
assuming that I will be using an admin user ?
I have a large number of users so it is not possible to subscribe the 
presence of all users. I also use xdb_sql but I could find the presence state 
stored in any table that I can directly query !

Best regards

Ramy