Information > Statistics > Diskspace displays how the system's individual partitions are working to capacity and have been in the past. The system partition should be working at a relatively constant to slightly increasing load. In normal operation, only a fraction of the capacity of the spool and log partition should be used.

If you notice a high usage of the partition for email, cache and backup and suspect a high volume of emails, you can use user statistics to find out which user is occupying how much space with their emails.