/usr/libexec/mail.local runs as root.  As such is can fill up a 
        mailbox on a quota'd filesystem, and keep going... Makes quota's
        almost useless in an ISP environment.

Fix: Here's a patch to mail.local (tested, it works) to cause mail.local
        to become the user before delivery is attempted, which will fail for
        overquota.  One note needs to be said about QUOTA's and mail, when
        one enables quota on a mail system sendmail must run a mail.local
        for each message (e.g. no lower m in the args F= in Mprog in 
        sendmail.cf).  If sendmail attempts multiple recipient delivery, and
        any recipient is over quota, the mail will be requeued for delivery
        again later.  Unfortunatly everyone else who was delivered to at the
        same time, without error, will keep getting copies of the message until
        the entire delivery attempt was a success.  This is an inherent flaw
        in mail.local, and not a result of this patch.
How-To-Repeat: 
        send a mail message..