Mail users and quota used question

The quota for my mail accounts defaults to 50mb

I check mail regularly and delete mail from the server at each connection.

For one of my accounts that had no mail left of the server it showed this:

50 MB <-- Disk Quota 49.68 MB <-- Quota Used 16 kB <--Mail Size

Why does the quota used not go down when mail is deleted?

Spam Assassin had shut off for that account and sending an attachment to that account bounced I assume because it is so close to quota.

How do I get the quota used to reflect what the mail size is or am I not understanding how this works?