Everyone in the CS department has an email quota for messages stored on the IMAP/POP mail server. The default quota for all CS users is 2GB. If you exceed your email quota, your mail will begin to bounce.
In most cases, you will receive an automatic warning email when you get near your quota. If you suspect you are over or near your quota, or if you just wish to monitor your usage, you can check check it by logging in to the CS Webmail system. The banner under your name in the Webmail client displays a bar showing how much of your quota you have used. You can hover your mouse over the bar to see actual usage. You can also see your email quota by logging in the CS Guide.