The idea of spam throttling came about after would-be spammers were easily circumventing (classic) tarpitting. A reasonable recipient limit in tarpitting must not adversely affect acceptable mail usage, so spam clients typically create multiple SMTP conne
qpsmtpd is a flexible smtpd daemon written in Perl. Apart from the core SMTP features, all functionality is implemented in small "extension plugins" using the easy to use object oriented plugin API.
SPAMCONTROL is an extension qmail. Though mainly used to filter and control unsolicited commercial E-Mails (UCE/SPAM), since release 2 it includes substantial EMSTP protocol enhancements for qmail.
This is an SPF implementation for qmail. SPF is something like a DNS based "reverse MX" system to designate permitted senders for mails depending on the domain name. The goal is to disallow sender address forgery.
Qmail-Scanner is an add-on that enables a Qmail email server to scan gatewayed email for certain characteristics (i.e. a content scanner). It is typically used for its anti-virus and anti-spam protection functions, in which case it is used in conjunction