HashCash is an anti-spam, anti-DoS system for email. The concept centers around the sender of the mail completing a computationally expensive task (in this case, generating hash collisions) that the recipient can verify very quickly. This allows the sender to "pay" for transport in CPU cycles, creating artificial scarcity. Programs are included to add payment to messages (for the sender) and to verify message payment (for the recipient).
mysqmail-postfix-logger is a (very small) daemon that logs all SMTP traffic to an SQL database. It splits the information by domain and user for the From and To fields, making it very easy to count all the SMTP traffic for a given domain name. DTC (Domain Technologie Control) uses it for logging the SMTP traffic.
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.
sendPGPMail.cgi is a small CGI script for sending an encrypted PGP email message to a predefined receiver from an HTTPS secured Web page. The typical use of this script is to provide the users of a secure Web site the possibility to send a secure email to a predefined employee of a company. The CGI script gets its input from an HTML form and passes it on to gpg. The message and its attachments are encrypted using the PGP public key of the predefined receiver.
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.
femail provides a sendmail-compatible commandline interface and forwards mail via SMTP. Unlike many other implementations, it is fully RFC 2821 and 2822 compatible. It embeds extra environment information from Apache in the header, if present, to help with tracking down abuse. It is written with security as a main design goal.
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.