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).
Grinch is a small Perl script which can be used to check whether a given address is an open mail relay host. It can operate as a daemon, and speaks the tcp_map protocol implemented in Postfix when it runs as a daemon. It can also be used in arbitrary shell script environments where the hostname is read from stdin and the error code is written to stdout, which allows it to be used with qmail and other MTAs.
dkimproxy is an SMTP proxy that signs and/or verifies Domain Keys Identified Mail (DKIM). It is designed to work with Postfix. It comprises two separate proxies: an "outbound" proxy for signing outgoing email, and an "inbound" proxy for verifying signatures of incoming email. With Postfix, the proxies can operate as either Before-Queue or After-Queue content filters.
vPostMaster is a complete email server for Linux. It is designed to be easily configured in all areas. It includes greylisting, whitelisting, blacklisting, and spam and virus filtering. It supports unlimited domains. It includes Webmail and POP3/IMAP as well as SMTP. Mailing lists and autoresponders are available in the commercial version. It is based on Postfix. The installation uses packages and takes only a few minutes. It features configuration through the Web of all portions of the system from a site-wide policy to per-user preferences and filters.
tMTA is a miniature Mail Transport Agent that vaguely emulates the command-line interface of Sendmail as used by many Unix programs such as mail(1). tMTA only supports local delivery to local users. It does not support SMTP or any form of inter-computer mail transport. It provides the barest functionality for daemons and users on a computer to communicate with each other using the common mail interface.