Maildrop is a powerful email filter/delivery agent. It supports mbox-style mailboxes and maildirs (a mail storage format used by Qmail). Its mail filtering language supports RFC822 parsing and GDBM/Berkeley DB database files. The maildrop package also includes a command-line tool to perform various operations on MIME messages.
DSPAM is a server-side statistical anti-spam agent for Unix email servers. It masquerades as the email server's local delivery agent and effectively filters spam using a combination of de-obfuscation techniques, specialized algorithms, and statistical analysis. The result is an administratively maintenance-free, self-learning anti-spam tool. DSPAM has yielded real-world success rates beyond 99.9% accuracy with less than a 0.01% chance of false positives.
John the Ripper is a fast password cracker, currently available for many flavors of Unix, Windows, DOS, BeOS, and OpenVMS. Its primary purpose is to detect weak Unix passwords. It supports several crypt(3) password hash types commonly found on Unix systems, as well as Windows LM hashes. On top of this, lots of other hashes and ciphers are added in the community-enhanced version (-jumbo), and some are added in John the Ripper Pro.
GNU m4 is an implementation of the traditional Unix macro processor. It is mostly SVR4 compatible, although it has some extensions (for example, handling more than 9 positional parameters to macros). GNU m4 also has built-in functions for including files, running shell commands, doing arithmetic, etc. Autoconf needs GNU m4 for generating `configure' scripts, but not for running them.
MailScanner is an email virus scanner, vulnerability protector, and spam tagger. It supports the Postfix, Sendmail, Exim, Qmail, and ZMailer MTAs, and the Sophos, McAfee, F-Prot, F-Secure, CommandAV, InoculateIT, Inoculan, eTrust, Kaspersky, Nod32, AntiVir, BitDefender, RAV, Panda, DrWeb, ClamAV, and other anti-virus scanners. It uses SpamAssassin for highly successful spam identification, and is designed to handle denial of service attacks. It will detect password-protected zip files and apply filename checking to their contents. It is very easy to install, requires no changes at all to your sendmail.cf file, is designed to be lightweight, and won't grind your mail system to a halt with its load. It can be integrated into any email system, regardless of the software in use.
The milter module for Python provides a python interface to Sendmail's libmilter that exploits all its features. Milters can run on the same machine as sendmail, or another machine. The milter can even run with a different operating system or processor than sendmail. Sendmail talks to the milter via a local or internet socket, and keeps the milter informed of events as it processes a mail connection. At any point, the milter can cut the conversation short by telling sendmail to ACCEPT, REJECT, or DISCARD the message. After receiving a complete message from sendmail, the milter can again REJECT or DISCARD it, but it can also ACCEPT it with changes to the headers or body.