Check out quotawarn: and quotawarnkb: in man imapd.conf. They result in IMAP ALERT messages being generated.
Note that these alerts are sent on SELECT, and it has been claimed that Outlook SELECTs a lot, making it unusable. I have not verified this.
Indeed, this can be a problem if you have a large number of folders. For example, I have about 30 folders under my Inbox for archived messages, various mailing lists, etc., and if I ever get near the quota, hitting Control+M to poll for messages is a really bad idea, because I'll get an alert dialog at least once for each and every folder -- and there's nothing you can do but keep hitting ENTER until it is done.
It can be annoying, but I'd wager that most people don't sit above that 90% threshold for long, before they clue in and dump some old mail.
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