VPopMail

This is an external software intended to make handling virtual domains easier. Supports Qmail and Postfix. See http://inter7.com/vpopmail.html

If the vpopmail database contains plaintext passwords, it can be used for non-plaintext authentication as well.

passdb parameters:

cache_key: If set, you can use auth_cache with VPopMail. See PAM for more information about it.
webmail=IP: If IP address is specified, connections from it are assumed to come from webmail and VPopMail's webmail usage restrictions apply.

vpopmail + mysql

Alternatively, you can use the SQL backend with the following configuration:

driver = mysql
#
connect = host=/var/run/mysqld/mysqld.sock user=vpopmail password=YOURPASSWORDHERE dbname=vpopmail
#
default_pass_scheme = PLAIN
#
password_query = SELECT CONCAT(pw_name, '@', pw_domain) AS user, pw_clear_passwd AS password FROM vpopmail WHERE pw_name = '%n' AND pw_domain = '%d'
#
user_query = SELECT pw_dir as home, 64020 AS uid, 64020 AS gid FROM vpopmail WHERE pw_name = '%n' AND pw_domain = '%d'

Most vpopmail installations use 89 as the uid/gid, not 64020.