Open5066 is an implementation of the NATO NC3A STANAG 5066 protocol stack for HF radio communications. It aims to implement all of SIS (Annex A), DTS (Annex C), and some of the application layer (Annex F) functionality such as HMTP. The concrete embodiment of Open5066 is the s5066d daemon.
libSmtpThread is a multi-threaded C++ library for sending mail via SMTP. The library provides a connection pool that initializes a set of threaded SMTP connections. Emails can be pushed into the pool, which accepts all mail and delivers them to a free connection. The delivery to the SMTP server is processed in a separate thread, so you don't have to wait for mail to be accepted. It has been successfully tested with Postfix.
DynamicRelay prevents your customers from sending spam, and authenticates users using POP or IMAP to send email in a pop-before-smtp fashion. You can use it with the wu-imap, dovecot, Courier, and Qpopper IMAP/POP servers, and with Postfix or Sendmail. You set the limit to the number of email messages a user can send per day. This is useful when you have users infected by viruses that send thousands and thousands of spam messages, or who send spam deliberately. You can also control how long users are allowed to send email after their last POP or IMAP authentication. Abuses are logged and emailed to you, and you can put your own static entries in the access file.
BackupMX consists of two programs, 'Store' and 'Forward'. Store accepts SMTP connections, takes all mail given to it, and stores it in a file. Forward takes that file and forwards it over SMTP to a fixed destination. This is code for integrating into your own projects rather than using as is. It is suitable for creating a disaster-recovery mail handler, but would also make a good basis for a spam filter, a tarpit, a honeypot, or a greylist filter. It is written in simple C, and has no external dependencies such as libsmtp.
postfix_memcached is a patch to Postfix (2.1 and 2.2) to allow it to use memcache as a map source. The memcache map type can be used to reduce the load on another map type, e.g. pgsql. By placing the memcache map before the pgsql map, a key not found in the memcache will fall through to the database. You'll need to figure out a way to reverse populate the cache from the database yourself. This map type can be seen as an alternative to the proxymap(8) service.