You will receive the following email,
Sent: Tuesday, December xx, 2010 x:xx AM
Subject: Your Mailbox Has Exceeded?
Your Mailbox Has Exceeded It Storage Limit As Set By Your Administrator, And You Will Not Be Able To Receive New Mails Until You Re-Validate It. To Re-Validate – > Click Here: System Administrator
This is a spam. When you click on the link, then you will be directed to a website that will ask the users to enter their username and password.
As an exchange admin, make sure that your users aware of this security threat to avoid disaster