CGI::ContactForm is a Perl module that generates a contact form for the Web and sends a message to a predestinated recipient. Unlike many form-to-mail programs, it's limited to this single purpose, but it allows you to set up a decently designed form with a minimum of effort. In other words, it's simple to use rather than flexible, even if there are quite a few customization possibilities.
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.
Dkfilter is a DomainKeys implementation for the Postfix mail transfer agent. It is implemented as two filters (or SMTP proxies). There is an "outbound" filter for adding DomainKeys signatures to outgoing email and an "inbound" filter for verifying signatures of incoming email. The filters can be plugged into Postfix as either Before-Queue or After-Queue Postfix content filters. For more information about DomainKeys, see http://antispam.yahoo.com/domainkeys.
SmartList is a mailing list manager. It is built on top of procmail and provides for the simple creation and handling of mailing lists, including fully automated subscription/unsubscription/help-request processing, intelligent auto-removal of addresses from the list that cause too many bounces, and a built in archive server (with MIME support).
SwenDeleter tries to identify email messages infected with the Swen worm in POP3 mailboxes and delete them on the server. It applies some heuristics to the headers and size of the messages, in order to avoid downloading the actual email, thus making retrievals less taxing. It has both interactive and nonstop modes.