Distributed Checksum Clearinghouse (DCC) is a system of clients and servers that collect and count checksums related to mail messages. The counts can be used by SMTP servers and mail user agents to detect and reject bulk mail. DCC servers can exchange common checksums. The checksums include values that are "fuzzy", or constant across common variations in bulk messages.
Enigmail is an extension for the mail client of Mozilla, Netscape 7.x, and Mozilla Thunderbird that allows users to access the authentication and encryption features provided by the popular GnuPG software. It can encrypt/sign mail when sending, decrypt/authenticate received mail, and import/export public keys. It supports both the inline PGP format and the PGP/MIME format, which can be used to encrypt attachments, and is cross-platform, although binaries are supplied only for a limited number of platforms. Enigmail uses inter-process communication to execute GPG to carry out encryption/authentication.
Email Security through Procmail (the Procmail Sanitizer) provides methods to sanitize email, removing obvious exploit attempts and disabling the channels through which exploits are delivered. Facilities for detecting and blocking Trojan Horse exploits and worms are also provided.
The Anti-Spam SMTP Proxy (ASSP) Server project aims to create an open source platform independent SMTP Proxy server which implements whitelists, Bayesian, and virus filtering to help stop unsolicited commercial email (UCE). Anti-spam tools should be adaptive to new spam and customized for each site’s email patterns. This easy to use tool works with any mail transport and achieves these goals requiring no operator intervention after the initial setup phase.
The GNU Talk Filters are filter programs that convert ordinary English text into text that mimics a stereotyped or otherwise humorous dialect. Some of these filters have been in the public domain for many years, but here they are provided as a single integrated package. The filters include austro, b1ff, brooklyn, chef, cockney, drawl, dubya, fudd, funetak, jethro, jive, kraut, pansy, pirate, postmodern, redneck, valspeak, and warez. This package provides the filters both as individual executables and collectively as a C library, so they can be easily embedded in other programs.
SPF is a new strategy for preventing junk mail. The present SMTP standard for email allows anyone to forge anyone else's email address. SPF verifies that the Sender address of an email message matches (according to some policy) the client IP address that submitted it. libspf2 is a complete and robust implementation of SPF which provides support for many MTAs. Support for new MTAs is in progress.
Mail2sh makes it possible to carry out shell commands by email. Email is sent to a particular user on your host and the commands will be carried out if the user and password given matches ones in /etc/passwd. Commands are executed with the user's privileges, and combined with a PGP module ensures a certain level of security for use. Note that the system is not natively encrypted, so use of an encryption mechanism is highly recommended for security reasons.
libSieve is an implementation of the Sieve mail sorting language originally developed for the Cyrus mail system. libSieve helps to bring mail sorting functionality into your mail server application without the need to reinvent the wheel. As a library, it is not intended for end users expecting a ready-to-run program.