A quota limits how much disk space is available for you to store Exchange data, such as your e-mail and calendar.
Every Exchange user has a 25GB quota.
In Outlook Web App, hover over the name of your mailbox and your current mailbox size and quota displays.
See one these docs for the answer, depending on your Exchange client:
ITS Exchange delivers two separate services - email and calendaring for users with sensitive research data and Voicemail. Determine which mailbox is over the quota.
For help with accessing your voicemail mailbox, see the Exchange Voice Mail page
(note: you may need to login with your uniqname and UMICH password)
Tips to control the mailbox size.
In Outlook Mail, click Search Folder and then click Large Mail. Delete the messages you no longer need or copy them onto your desktop and then delete the message.
Empty your Junk E-mail and Deleted Items folders.
Sort your Sent Items folder by size and remove large messages you no longer need.
Delete attachments. If you believe you'll need the attachment, save it to your desktop first.
Outlook (Win): Open the message, right-click the attachment, and select Remove. Save the message when prompted. There will be no indication that there ever was an attachment.
When you are within 1% of your quota, you will receive a warning message. For example, for the basic 500MB quota, you will receive a warning when you have used 450MB.
If you reach 99% of your quota, you'll receive a daily warning message until you reduce your account's size.
Should you exceed your quota, you will not be able to send e-mail until you have removed enough messages to get below your quota. You will, however, continue to receive mail, but you will not be able to respond or create new messages.