elspy is an extension to the Exim MTA that allows Exim users to supply a local_scan() function written in Python instead of C. (Exim's local_scan() API lets sysadmins supply site-specific code for approving/rejecting email messages at SMTP time.)
An open project for a number of activex controls for use in the Windows environment. The controls have been sold commerical before by Eurosource (http://www.eurosource.se) but are now free complete with sourcecode.
authd mail policy server; this server talks to postfix, sendmail, and anything else that wants to link to libauthd.a. The code has been stable for years, but the documentation brings us from Mature to Beta. This project has had no changes in 2+ years oth