MilterQuota is a milter (mail filter) for sendmail using libmilter. It implements quotas for inboxes without any influence of the amount of disk space the user can fill. There is one central configuration file where you can enter pairs of user/max.inboxsize and you can also enter a default value for all users. This makes a site-wide configuration easy. An additional feature is the override address: as an administrator you can still send important mail to users with full inboxes.
