Perdition is a mail retrieval proxy that allows users to connect to a
content-free POP3 or IMAP4 server that will redirect them to their real
POP3 or IMAP4 server. This enables mail retrieval for a domain to be
split across multiple backend servers on a per user basis. It can also
be used as a POP3 or IMAP4 proxy in firewall applications.

Perdition supports arbitrary library based map access to determine the
server for each user - POSIX regex, GDBM, PostgreSQL, MySQL, NIS and
OpenLDAP libraries ship with the distribution.

The use of perditon to scale mail services beyond a single box is discussed
in a paper the author wrote on high capacity email, so be sure to check the
web page.