Amber sits in the tcpserver chain for qmail and implements an "amber list" for incoming mail. This neither accepts nor rejects connections, but implements a number of timing-related tricks to fake out spam and virus software that doesn't implement SMTP correctly. For example, it can defer requests from new IP addresses for a few minutes before it starts passing connections on to smtpd, pause and check for programs that send data before the HELO, and otherwise make things rough for bandit bulk mailers.
Wumi is a sendmail milter using the py-milter. It rejects mail containing certain (configurable) attachment filenames (.exe, .pif, etc.) or containing a virus (checked by calling a single-user license command line scanner) by answering the SMTP DATA step with "554 5.7.1". This way, the mail server does not have to generate senseless bounces.
PHP MIME Mail decoder class is a PHP class to parse and decode MIME email messages. It supports decoding single RFC 2822 MIME messages or archives that aggregate multiple messages in the mbox format. It can decode messages read from files or data strings. The bodies of multipart messages can be decoded into distinct sub-messages. Optionally, the message body data can be saved to separate files, so the class can handle messages larger than the available memory. The parser can tolerate some syntax errors in malformed messages that are not compliant with RFC 2822.
fdm (fetch and deliver mail) is a simple, lightweight replacement for mail fetching, filtering, and delivery programs such as fetchmail and procmail. It can fetch using POP3, POP3S, IMAP, IMAPS, or stdin, and deliver to a pipe, file, maildir, mbox, or SMTP server, based on a set of regexps. It can be used for both single user and multiuser setups, and is designed with privilege separation when running as root.
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.