RFC 2015 defines a proposed Internet standard for sending PGP-encrypted email. This PGP/MIME has been incorporated into several MUAs such as Mutt and (with the AEGYPTEN project) KMail. However, a lot of email clients still don't support it. pgp-mime-handler can decrypt/verify such messages via a pipe, so it can be used as an email filter in many programs or scripts.
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.
MailStripper Pro is a mail scanner that aims to remove spam and viruses from incoming mail. It recognises and decodes many obfuscation techniques used by spammers and uses several different mechanisms to recognise spam which are immune to Bayes poisoning, and has achieved real-world success of over 99.8%. It is MTA-independent and does not implant itself into your mail server. Instead, it functions as a proxy, allowing the MTA to reside on a different server.
P3Scan is a full transparent proxy server for email clients. It runs on a Linux box with iptables (for port redirection). It can be used to provide email scanning from the Internet to any internal network, and is ideal for helping to protect your "Other OS" LAN from harm, especially when used in conjunction with a firewall and other Internet proxy servers. It is designed to enable scanning of incoming email messages for viruses, worms, trojans, spam, and harmful attachments. Because viewing HTML email can enable a spammer to validate an email address (via Web bugs), it can also provide HTML stripping.
Logrep is a secure multi-platform tool for the collection, extraction, and presentation of information from various log files. It features HTML reports, multi-dimensional analysis, overview pages, SSH communication, and graphs, and supports 25 popular systems including Snort, Squid, Postfix, Apache, Sendmail, syslog, iptables/ipchains, xferlog, NT event logs, Firewall-1, wtmp, Oracle listener, and Pix.
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.
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.