amavisd-new is an interface between message transfer agent (MTA) and one or more content checkers, e.g. virus scanners, SpamAssassin, etc. It is a performance-enhanced and feature-enriched version of amavisd (which in turn is a daemonized version of AMaViS or amavis-perl). amavisd-new is normally positioned at or near a central mailer, not necessarily where user's mailboxes and final delivery takes place. If you are looking for fully per-user configurable and/or low-message-rate solution to be placed at the final stage of mail delivery (e.g. called from procmail), there may be other solutions more appropriate for your needs.