To keep our systems healthy and your account safe, each Google Apps Gmail account has a limited amount of storage, currently 25 GB. The limit does not inhibit normal email use but is designed to help identify any misuse of Gmail.
If an account reaches the storage limit, a user cannot send or receive any new mail and may experience general account degradation. Specifically, the sender will receive a bounce stating "The email account that you tried to reach is over quota."
See the Your storage limit article for instructions on deleting messages to reduce storage use and the Solutions section below for workarounds to a full mailbox.
The storage available in each account is tracked by an indicator found at the bottom of every Gmail page.