The Cyrus IMAP server is generally intended to be run on sealed systems, where normal users are not permitted to log in. The mailbox database is stored in parts of the filesystem that are private to Cyrus. All user access to mail is through the IMAP, POP3, or KPOP protocols. The private mailbox database design gives the server large advantages in efficiency, scalability, and administratability. Multiple concurrent read/write connections to the same mailbox are permitted. The server supports access control lists on mailboxes and storage quotas on mailbox hierarchies, multiple SASL mechanisms, and the Sieve mail filtering language.
The DBMAIL package replaces the normal UNIX mailing system. All email and user data is stored in a database. You can create an unlimited number of email accounts, which can be checked using the POP3 or IMAP protocol. Users can maintain their own set of email addresses. It is more scalable, more secure, and faster than traditional mail systems. DBMAIL has storage drivers for PostgreSQL and MySQL, and it has authentication drivers for PostgreSQL, MySQL, and LDAP.
DBOX is a free communication package for ISPs. It supports a large number of message formats such as RFC/UUCP, ZConnect, Janus/Janus2, Alphabox, and GSbox. It is designed for large installations and includes a WWW package with an email/HTML gateway, an HTML online shop, support for POP3/IMAP4, and personal homepages with HTTP/FTP upload.
Deliver is a program for reliable and flexible local mail delivery. It's a perfect adjunct to your mail transport agent of choice (such as Sendmail or qmail). You can use it to kill spam, drop messages from specific senders, add a [LISTNAME] prefix to the subjects of messages that are delivered through mailing lists, and lots more.
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.