I saw mentioned on one of the forums that there should not only be a per-user limit but also a per-mailbox limit. Because most of the time it's the individual mailbox that gets hacked, not the entire user account. So having both limits, set independently, would be really really useful.
This is required because other ISP's and Mail Servers on the recipient side don't measure email limits per day, they measure per hour. So DA must also have the limits per hour.