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.
fakehermes is a small, lightweight fake SMTP server. It's useful as a way to avoid spam by configuring your domains to point their secondary SMTP server to a fakehermes server. It will log not only the email messages involved, but also how they are trying to send email and whether they should be considered spam or not. This information can be used to "profile" the spammers that send you spam and can allow you to configure or tune other spam filters. It supports SPF and RBL checking.
MFilter is a replacement for maildrop/procmail and fetchmail/getmail. It is an email retriever that supports Maildir or command delivery and processing. Its secondary goal is to facilitate the life of the network administrator by centralizing the downloading, filtering, and classification of mail in a simple and flexible way. Some of its features include XML file based configuration and logging of each action taken.