qpsmtpd

It is no secret that our service is based upon a collection of open-source software, and the major component of that software is the open source SMTP proxy server qpsmtpd.

This is a small and flexible SMTP engine written in perl which implements the minimum necessary to present itself as an SMTP server, the actual work of the server is completed by a collection of plugins.

The core of our service is a collection of custom plugins which implement a range of different tests against incoming mails. The majority of these plugins are very specific to the setup of our service, but some of are more general purpose, and in the spirit of sharing here they are:

check_system_load

This plugin provides a simple means of coping with a large amount of incoming mail. If the system load rises above a particular threshold then further incoming transactions are denied.

spam_gourmet

This plugin provides a means of distributing email addresses which have built-in self-destructs.