How to edit EmailAccount usage quota limit

Hi everyone, I have a user that has a disk quota limit of 102400 MB of space, and Max Quota for email address Unlimited. But when i go to Disk Usage i see that the bar for "Email Accounts"" is full ( 6,797.12 MB ) . The user keeps getting email from the system saying this: "The domain "xxx.it" has reached their disk quota. The account currently uses 97.73% of its disk capacity. " But Disk usage says: 13,921.25 MB total disk space used. So I suppose it is referring only to the Email Accounts" part. How can i increase the limit ?