When you check your email using a program like Outlook Express using the POP3 protocol, the messages are downloaded from the mail server to your computer and therefore do not count towards your quota. Reasons for your box to exceed the quota include:

1. Email attachments: Large email attachments may lead to a mailbox exceeding the quota. Generally speaking, email messages without attachments are less then 10 Kilobytes; however, an attached image, program file, or multimedia file can sometimes be larger than 5 Megabytes!
2. Copies of messages are left on server: If you have configured your email program to leave a copy of your email messages on the server, your mailbox becoming over quota.
3. Spam Folder: If you use the Expedient Spam Filter, messages in your Spam folder count towards your quota. If you receive a lot of spam, you may need to manually remove messages from the Spam folder