Right Zu, but some people choose to save certain mails from certain people, hence they get to choose which ones are deleted etc.
I.E I have 20 or so mails I'd like to save, i'd be rather pissed if they get deleted and the junk ones stay.
Hence it's better for the player if their mailbox is full, maybe they'll get an Event sent to them when someone can not deliver a message due to a full inbox.
Except Staff off course. They should always be able to send mail.