Python SRS Library is a Python translation of the Perl reference implementation for SRS (Sender Rewriting Scheme) with easy to install packaging and built-in sendmail integration. SRS is needed when forwarding mail for SMTP authorization schemes that publish which IPs are authorized to send mail from a domain (for example SPF - Sender Policy Framework). It can also detect forged SMTP bounces by signing the return path for outgoing mail.
WBL is a really simple tool that can be used in a .qmail file for managing a white/black list of email addresses. It serves as a pre-filter to allow known-good friends and associates bypass the usual gauntlet of filters. Note that WBL is not intended to be a spam filter, it is simply an easy-to-use whitelist/blacklist management tool that serves to keep email that you really want to see out of your normal filters.
smtpd is a lightweight mail server whose sole purpose is to receive incoming messages and deliver them to local users. Its main "feature" (or non-feature, really) is to be completely unable to relay messages to adjacent servers: if an incoming message is not addressed to a local mailbox, smtpd will reject it. Another convenient feature is its automatic grey-listing of clients that allows you to efficiently reject spammers based on their behaviour. It can handle incoming email traffic for an unlimited number of domains (i.e. act as a MX for these domains) without the need for any configuration (other than creation of users' mailboxes).
Inspector is a little Python script for the Postfix mail server that helps with detection of NDR (Non Delivery Report) spam messages. It adds a watermark to every email message being sent out of the defined "mydomains", and implements a mechanism for checking for the presence and validity of such watermarks in every NDR message. It means that you can check if the NDR message is valid or not (i.e. it is NDR spam).
eXtremail manager is a PHP-based SMTP/POP server manager, specifically for the eXtremail server. It offers the server admin many ways to monitor server activity, and add, edit, or delete users and domains. It also allows the main admin to specify admins per domain, which allows those "mini-admins" to administer their own domains, and none of the other ones.
PMilter implements Sendmail's "milter" mail filtering protocol in Perl, allowing scripts direct access to the SMTP transaction in progress, rather than filtering later. It can function as a drop-in replacement for Sendmail::Milter, though PMilter does not require threads or the "libmilter" API to be available.