POP3 and the ISP

So you have hit on one of the reasons ISPs don't like pop3. People get mail into their account and leave it on the server even when they don't want to keep an copy. Causing filled mail boxes.

As it is clear that google does offer a system of notification for full boxes, and having worked an a team running a large (50k+ active users) mail systems quota notification systems are a pain both technically and administratively.

The better solution than a pop3 desktop client for such an inherently web based services is one of the many nice Firefox plugins to help you keep on top of your gmail.