Pigeonhole is the name of the project that adds support for the Sieve
language (RFC 5228) and the ManageSieve protocol (RFC 5804) to the
Dovecot Secure IMAP/POP3 Server.

Sieve language specification and scripts provide mail filtering
capabilities on the server side for individual users. Each user can
edit and upload (using ManageSieve) sieve scripts to filter mail based
on supported Sieve language extensions within pigeonhole.

The Pigeonhole project provides Sieve support as a plugin to Dovecot's
Local Delivery Agent (LDA). The ManageSieve protocol is provided is an
additional service, next to Dovecot's own POP3 and IMAP services.

To configure pigeonhole, use sieve and managesieve config files
installed with dovecot documentation. Copy 20-managesieve.conf and
90-sieve.conf files to /etc/dovecot/conf.d and edit them according to
your needs.

For additional help with configuration, see:

  http://wiki2.dovecot.org/Pigeonhole/Sieve
  http://wiki2.dovecot.org/Pigeonhole/ManageSieve