POP3 Settings

You know, when you set up your POP3 account in Outlook and I dare say in Thunderbird too (don't use it, don't know) that allows you to delete the server copy of your mail when you download it to the client.

That would sort the issue, surely. Plus it avoids Google sending out emails about your remaining space and the need to look at the web interface to check remaining space because, unless you receive a limit-busting email in one hit, it would remain constant from one attempt to check mail to another.