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
mails based on supported Sieve language extensions by pigeonhole.

Pigeonhole project provides Sieve support as a plugin to Dovecot's
Local Delivery Agent (LDA).

Configuration:
sieve and managesieve config files are placed with dovecot
documentation.  Copy over 20-managesieve.conf and 90-sieve.conf
to /etc/dovecot/conf.d and edit accordingly.

For additional configuration help, see:

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