Free filter software for linux

WebAvis is a Web frontend to amavisd-new written in PHP.

It allows owners of a mail account to manage their amavisd-new parameters, like spam scores, white/black lists and filter behavior.

Constrictor is a Python module for parsing, filtering, and delivering mail as a Mail Delivery Agent (like Procmail). Constrictor provides the functionality to easily write a MDA in Python rather than reading some configuration file of its own syntax.
This has the advantages that configurations are readable and self-explanatory (at least, to anyone who knows Python) and that users have the full flexibility of the Python language at their disposal. Constrictor has the ability to deliver to maildir and Unix mbox format mailboxes, as well as bounce, forward, or reject messages.
Constrictor is currently beta software, at best. Im always very leery of software that messes with my mail, since one runs the risk of not only losing messages but, potentially, bouncing mail to great embarassment and chagrin. That said, I am currently using Constrictor on my own mailbox full-time, but, as always, your mileage may vary.
Installation
Constrictor is a single Python module that should be placed either in the directory you intend to invoke your MDA from, or in your Python library directory. The only non-standard module it relies on is the spamd module from the SpamAssassin distribution (available at http://spamassassin.apache.org/full/2.6x/dist/contrib/spamd.py), and only then if you intend to use the "spamassassin()" function to filter your mail (the module is loaded only when that function is called). Constrictor should work on all versions of Python greater than 2.3; it has been tested primarily on 2.3.4, 2.3.5, 2.4.1.
The spamassassin() function connects to a (optionally specified) spamd server (it defaults to localhost). If you do not have a spamd server, you cannot use this function.
For Postfix and Sendmail, edit your ~/.forward to include "|/path/to/filter.py". For more information on invoking this script as an MDA,see your MTAs manual or ask your administrator.
Enhancements:
- This release adds locking code to duplicate database (pickle file used to recognize duplicate messages) access.
- This release contains no known stability or delivery reliability issues, and thus is considered stable.

XMMS hiss filter plugin can dynamically filters background hiss.

SSpamM comes from Semis Spam Milter is a spam filter for Sendmail that utilizes spambayes, heurestic spam filtering, and virus scanning (BitDefender, not included).

smf-clamd is a Sendmail milter for the ClamAV (Clam AntiVirus).
Sendmail filter for ClamAV aims to be lightweight, reliable and simple. Sendmail filter for ClamAV is written in C.
Main features:
- It has a hosts/networks whitelist;
- It can scan messages less than a defined size only;
- It rejects infected messages at the SMTP DATA stage;
- It can add an information header with scan results to scanned messages;
- It can log all milter activities through the syslog service.
Advantages:
- Small code;
- Fast work;
- Stability (production quality);
- Few system resources are required;
- No temporary files are created.
Enhancements:
- A workaround for the Sendmail socket unsafe error on some platforms was implemented.
- Clean email messages logging was removed.
- Configuration file, default user, and working directory were changed.
- The format of log records was changed.
- Some samples of start-up scripts were added.
- The new TODO tasks were added.
- Cosmetic enhancements were made.

Compact Filter is a network packet filter for Linux. It features an easy-to-use compact filter representation and high performance.
The main features are: easy to use interface, compact filter representation (memory efficient), and very high performance.
A difference between CF and other firewalls, such as Netfilter, is its representation of the ruleset. In CF the user writes a filter which is then compiled and optimized in user-space. The optimized filter is then loaded into the kernel (using netlink). Consequently the in kernel packet filtering code only needs funtionality for setting up a filter and filtering packets, while the more complex code of optimizing the filter remains in user-space.
Because of the continuous increase of the bandwidth and the security threats, firewalls have to evolve towards more efficient filtering schemes. The truth is that the existing scheme does not scale so well with the combined growth of bandwidth and rulesets.
Our aim is to try another approach to perform packet filtering where we minimize the complexity of the filtering process. This result in, both, a reduction of the required CPU power to filter packets and a simpler (and smaller) kernel code. Of course, we are pushing all the smart and complex part out to the user-space, but, developing in user-space is much simpler and safer.
Enhancements:
- changed -d option to delete all filters
- added -D option to delete a specific filter
- changed default policy to permit rather than deny
- added -l option on flex to support --yylineno on older versions of flex
- new and improved install guide

yaspi (Yosis Anti-Spam POP3 fIlter bot) is a POP3 mail-scanner specifically made to combat mail bombings caused by viruses (i.e. Swen). It connects to the POP3 server, inspects the messages, and deletes the infected ones by itself automatically without ever downloading them.
yaspi most interesting feature is that it can automatically send abuse reports to the ISPs where the mails were originated. This way, most infected machines will be either disinfected or disconnected from the Internet (and so, sending infected mails!).
Main features:
- its free, distributed under the GNU GPL
- runs under Windows and Unix/Linux (maybe under Mac OS X, but untested)
- virus detecting rules are fully customizable
- integrated with Ricochet to send abuse reports
Enhancements:
- yaspi was killing its own messages, new rule added to let them pass
- new GUI available, still alfa, have several know bugs (but its not going to corrupt your mailbox, I hope!).