JSpamfilter is software that intercepts connections from remote Internet mail servers before they can connect to your mail server. The originating IP address for the inbound connection is then checked against one of the DNS-based SPAM Blackhole (DNSBL) lists maintained on the Internet by various organizations. If the check passes, the connection is allowed to proceed to your mail server; if it fails, the connection is told that the mail server is unavailable, then dropped.
purepost is an interface between Courier-MTA and Sophie for virus scanning. You can have it check mail for all your users, or just a few. Using courier's dot-courier functionality, this script intercepts an email before it hits the mailbox, creates a temporary file, tells sophie to scan that temporary file for viruses, and if it finds a virus, denies delivery and notifies the sender (otherwise it allows the message to be delivered).
QUICA is a Web interface to manage various aspects of email administration. Working as a courier-mta frontend, it can make a quick and easy to use email server. Tasks like creating accounts and forwards can be done easily. It manages domains separately, allowing the delegation of each domain to different people. It's a great tool for your customers if you are a service provider, but also works well for a single domain. Quotas for each account are managed, the number of email messages and hard disk space can be controlled accurately. Internationalization is provided (with some languages included) and you can add your own. The interface allows you to delegate the management to non-sysadmins.
The inline_smtp Perl script was created to allow filtering of incoming mail on a separate server that houses the mailspools without running a full-blown MTA. While it's possible to configure another MTA, e.g. postfix, to relay all mail to a smart host and do filtering through postfix, the queue can get behind easily, and on a busy host thousands of messages can get queued in a few hours causing delays in delivery from a few minutes to several hours. By filtering while redirecting, mail arrives without any delays, and can be filtered by a seperate host adding headers or content reducing the load on the final mail host.
femail provides a sendmail-compatible commandline interface and forwards mail via SMTP. Unlike many other implementations, it is fully RFC 2821 and 2822 compatible. It embeds extra environment information from Apache in the header, if present, to help with tracking down abuse. It is written with security as a main design goal.
elspy is a layer of glue code that enables you to write Python code to scan email messages at SMTP-time with the Exim MTA (mail transport agent). It also includes a small Python library with common mail-scanning tools, including an interface to SpamAssassin and a simple-but-effective virus detector.