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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.
No you can't do that and you will have to upgrade to the Google Apps for Business in order to get more space.
From the FAQ for Google Apps:
Can I have a subset of employees on Google Apps for Business and others on Google Apps?
Google Apps does not support Google Apps and Google Apps for Business user accounts as part of the same organization's Google Apps account.
Yes
In your Gmail settings, under "Accounts and Import", if you have multiple "send mail as" accounts set up you can choose to always default to your Gmail account or to respond with the account to which the message was sent.
Best Answer
I'm not aware of such a feature.
However, the current quota and usage is exposed via IMAP and used by some IMAP clients (eg Thunderbird) when you ask it for current quota information. I would have expected this information to be exposed via the Outlook Connector also. Does Outlook have a function somewhere to show the current usage info?
You could also implement something yourself using IMAP OAUTH if its really important to you that it sends email. It would be a trivial thing for someone to write a Marketplace app to do the same thing.