Limiting inbox

["Adair Vieira Júnior." <vieirajr MailAndNews com>]

I'm trying to limit the size of inbox of users, the messages received are
stored in /var/spool/mail/$user.

Quota for the user prevent that he writes on the homedir but he don't
receive a mail informing that his limit is finished and that his inbox
is full.

How to I manage this? Which the program that controls the inbox?

I have procmail-3.13.1/sendmail-8.9.3/imap-4.5 and kernel-2.2.19.

Thanks for support.