Comment by kraptv

Sieb is a mail filter/delivery system intended to work with Qmail using the Maildir format for delivery.

It's new but with 25 years of use case exposure of procmail and maildrop for filtering/processing/delivering mail.

It's not a swiss-army solution for everything but should be worthwhile for whole sites or individual users (user rules override)

This isn't my project but I thought it was useful and new in a fairly quiet part of the Internet.