Yes, Unity warns you when you approach limit 1, when you get to limit 1 and when you get to limit 2 (1=can't send, 2=can't send or receive).
Messages go to the unaddressed messages DL only if it's an outside caller, just to be clear - subscribers leaving messages for other subscribers will get an NDR back.
And also just to clarify... Unity has NO control over this. Exchange will simply not allow the messages to be delivered to a person who has reached limit 2, we can't get around it.
Also, in Unity 3.1(5) and later you can have Unity check the limit of a target mailbox when outside callers are leaving messages for a single user - if the limit 2 is reached we wont allow them to record a message - this doesn't work, of course, if they're leaving a message for a call handler set to deliver messages to a public distribution list but gets most of the cases you'd run into. This is a registry setting - you can turn it on in the Advanced Settings tool.
Finally, you may be interested in checking out the message store manager tool here:
http://www.answermonkey.net/App_MSM.htm
It is designed to help VM only installs in particular manage their mail limits and understand where message store space is being taken up, actively archive/delete messages based on rules etc... there's a training video out there for it as well, folks may find it handy for dealing with such scenarios.
