In a typical messaging environment, you'll have all sorts of users. Some of them seem to handle e-mail very efficiently and manage to keep their mailbox size very low. It could also be they aren't receiving tons of e-mails. Others do have the tendency to keep everything, therefore possibly using more space than you initially anticipated and sized for.

Even though every design should calculate for additional storage (up to a certain level) without getting into issues, it's important that you keep an eye on the amount of storage that you have left. One way to control how much storage a database can use on disk is to limit the amount of storage mailboxes within that database can use. As such, you can safely predict the maximum database size without getting into storage space related issues.