Licence management with XMON

As a newbie to NOD products I want to understand how licencing functions with XMON. Given that licencing for the Exchange module is based on the number of mailboxes protected, how do you manage this given the the number of email addresses can change quite rapidly compared with the user model used for the other modules of NOD. Do you have to pay for a larger number of licences than you currently need to allow for this rise and fall or is there some flexibility built in the the number purchased. Also, are all mailboxes counted? eg. each group can have a mailbox etc.

Cheers

Geezergeek