maildirproc is a program that processes one or several existing mail boxes in
the maildir format. It is primarily focused on mail sorting — i.e., moving,
copying, forwarding and deleting mail according to a set of rules. It can be
seen as an alternative to procmail, but instead of being a delivery agent
(which wants to be part of the delivery chain), maildirproc only processes
already delivered mail. And that's a feature, not a bug.