ACL Policy Daemon communicates with the Postfix MTA using the Policy Delegation Protocol, implementing an ACL (Access Control List) system. Key features: greylisting with flexible storage using memory for fast responses or disk for high persistence, SPF validation, control of messages by day/time, variable message size limits per domain or email, multiple RBL checking, and various ACLs available to use and combine. The configuration is simple and intuitive.
Bulk Mailer helps speed the delivery of an email message to many recipients using Sendmail. It's best used in parallel with a mailing list manager such as Majordomo. For input, it takes a file of recipient addresses (one address per line) and a message (with headers already attached) to be sent to the recipients. It sorts the recipient list by reversed domain (so similar ones sort together), splits up the recipients into several groups containing no more than N domains each, creates an SMTP envelope for each group of recipients, and feeds each envelope to sendmail.
CAKE (Key Addressed Crypto Encapsulation) is a bunch of Python and C++ for implementing the CAKE protocol. The CAKE protocol is a protocol in which all messages have source and destination addresses that are public keys. This is in the same way that IP packets have a source and destination address that is an IP address.
Citadel is an advanced messaging and collaboration system for groupware and BBS applications. Users can connect to Citadel using any telnet, WWW, or client software. Among the features supported are public and private message bases (rooms), electronic mail, real-time chat, paging, shared calendaring, address books, mailing lists, and more. Unlike other collaboration servers, Citadel provides its own data stores and is therefore extremely easy to install; you don't have to "bring your own" email and database because they're built in. The server is multithreaded and scalable. In addition, SMTP, IMAP, and POP3 servers are built-in for easy connection to Internet mail. Citadel is both robust and mature; it has been in production since 1987.
The Courier mail transfer agent (MTA) is an integrated mail/groupware server based on open commodity protocols such as ESMTP, IMAP, POP3, LDAP, SSL, and HTTP. Courier provides ESMTP, IMAP, POP3, Webmail, and mailing list services within a single consistent framework. Individual components can be enabled or disabled at will. Courier implements basic Web-based calendaring and scheduling services integrated in the Webmail module.
The Courier IMAP/POP3 maildir creation patch adds the ability to automatically create a user's home directory when he/she is successfully authenticated for the first time. It works by calling an admin-created script that takes care of creating the directory. It is quite useful in combination with authentication-modules based on network databases like MySQL or LDAP.
dkimproxy is an SMTP proxy that signs and/or verifies Domain Keys Identified Mail (DKIM). It is designed to work with Postfix. It comprises two separate proxies: an "outbound" proxy for signing outgoing email, and an "inbound" proxy for verifying signatures of incoming email. With Postfix, the proxies can operate as either Before-Queue or After-Queue content filters.
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.
DynamicRelay prevents your customers from sending spam, and authenticates users using POP or IMAP to send email in a pop-before-smtp fashion. You can use it with the wu-imap, dovecot, Courier, and Qpopper IMAP/POP servers, and with Postfix or Sendmail. You set the limit to the number of email messages a user can send per day. This is useful when you have users infected by viruses that send thousands and thousands of spam messages, or who send spam deliberately. You can also control how long users are allowed to send email after their last POP or IMAP authentication. Abuses are logged and emailed to you, and you can put your own static entries in the access file.