SymptomsUser1 uses Lync 2010 client which is hosted on Windows XP. They add a Lync 2010 user (User2) from another subscription using its SIP address. The status of added user is always 'Offline'.
When User1 tries to send instant message to User2, Lync client says that 'This message was not delivered to <User2> because this person is unavailable or offline'.
The same issue occurs when users from other subscriptions try to communicate with User1 that is hosted on Windows XP.