I manage a domain on Google Apps for a lot of heavy email users who mostly use Outlook and rarely login to the Gmail application.
I know there is a quota monitor at the bottom of the page when you login, but can Gmail be setup to send warning emails you you reach 80% or something similar?
That way they know when they should start some housekeeping.
I have searched the Google support pages and have not been able to find anything about emails being sent.