Broken mailbox quotas

[#1]

I wonder if anyone has seen this problem before.

I have been receiving warnings weekly about having too much stored email. At the beginning of March I received one of these and did some housekeeping, including deleting one mailbox in its entirety, containing about 100MB of mail. The following week, I still received a warning, the total had hardly changed. I have since tried recreating the same mailbox, which of course now shows as empty, and deleted it again. This 100MB is counting towards my quota and I don't know how to clear it.

[#2]

Perhaps swap to POP3 format, and be in control of your storage on your local storage.

[#3]

Not my preference. I like to be able to access from multiple devices until such time that I copy the messages to my local server.