Decimail Server is a mail server supporting IMAP and SMTP that uses a PostgreSQL database to store messages. Mailboxes are defined using SQL queries, allowing complex client-independent automatic message filtering and categorisation. For example you can partition your messages chronologically, by correspondent, and by mailing list at the same time, with each message appearing in several mailboxes. You can also use PostgreSQL's text indexing extension for fast keyword searching. Decimail also features anti-spam processing of outgoing messages.
dict_pgsql is a dictionary for the mailer postfix. It is able to get all virtual users out of a postgres database; this is very helpful if you have a large number of users who make updates on their aliases quite often. You don't have to extract all the values into system files; the entries will be fetched directly from the database.