Exchange 2003 or 2007?
If 2003, doing this natively requires serious modifications to the Exchange DLL files, which you don't want to do as you can easily corrupt things. On 2003, you'd need an add-on, like the Quota Message Service, to make the changes for you. See
http://www.msexchange.org/articles/Customizing-Mailbox-Limit-Warnings-Quota-Message-Service.html.
-Mat