Plugin to implement the 'greylisting' algorithm proposed by Ed Harris in http://projects.puremagic.com/greylisting/. Greylisting is a form of denysoft filter, where unrecognised new connections are temporarily denied for some initial period, to foil spammers using fire-and-forget spamware, http_proxies, etc.
This module was written by Gavin Carr, and the current version is available (together with other modules written by Gavin) from http://www.openfusion.com.au/labs/qpsmtpd/.
The version on this page contains two improvements over the version it is based on:
- There is a whitelisting mechanism for IP addresses. Gavin has since added a similar, but not identical mechanism (his "whitelist_soft" module). Whitelist_soft is a generic whitelisting solution which affects all modules which check for the notes it adds. My mechanism is specific to this module (so you can exempt a server from greylist checking, but still subject it to other checks.
- A new option per_recipient has been added. If it is used, greylisting is enabled only for recipients which want it. Again, whitelist_soft offers similar functionality, but on a global basis.
The following MTAs do not handle temporary failures correctly and hence cannot send mail reliably to an MTA which implements greylisting: