To know how well the end users are behaving and if they have a lot of mailbox volume, it is usually checked from the Exchange administration console.

We're going to “Servers” > “EXCHANGE SERVER NAME” > “First storage group” > “Mailbox storage” > “Mailboxes”, and it will be clearer than my example, with all the users we have, their total number of emails, their size in KB, when their last access to the email was and with which protocol…