mapSoN is an anti-spam system that uses an approach which is entirely different than other systems. Instead of trying to recognize spam by the IP address of the SMTP dialog's peer or by certain patters in the mail's body, it uses the sender's email address to decide whether the e-mail is delivered to your mailbox or not: Any email that comes from a "known" address may pass, but any email that comes from an email address seen for the first time needs special confirmation before it may pass. "Special confirmation" means that mapSoN will generate an MD5 checksum of the to-be-confirmed email and store the email in a temporary spool directory. Then it sends a request for confirmation to the address from which the mail was coming. In this request, it will include the MD5 checksum and ask the recipient to reply back and to quote that MD5 hash. Once it sees that MD5 hash again, it considers that a confirmation of the original email, delivers the deferred email from the spool to your mailbox, and adds the sender's address to the database of known addresses, so that the next time he tries to contact you, his mail will pass through immediately. This heuristic catches almost any spam email, because spammers have to fake their sender addresses in order to avoid being held responsible for their abuse. Hence, their address will most likely not be in the database of known addresses, nor will they ever receive the request for confirmation email.
The spasm anti-spam milter is a spam filter for sendmail 8.12+ (with libmilter support). Its current features include logging and quarantining of rejected spam, fully virtualised settings for domains, individual whitelists and blacklists, and over two dozen filters, which can be set individually on a per-domain or per-email address basis, including local (system) blacklist, rDNS resolution, HELO/EHLO verification, envelope sender address verification, numerous DNS-based lists, whitelist-only mode, and auto-blacklisting via spamtrap addresses and unknown user rejections.
Scriptid is a program and a library that can be used to determine whether a given text file contains code of a specified programming language. The current release can tell whether a file contains vbscript or not. It should be possible to extend this to any number of other languages. It is important to also download the latest neural network weights update file.
mxallowd is a daemon for Linux Netfilter (using libnetfilter-queue) and BSD pf (using pflog) which implements a slightly improved nolisting mechanism. It requires your name server to be configured to return two MX IP addresses, and the one with higher priority must not run a mail server on port 25. mxallowd blocks attempts to connect to the mail server unless the sender tries to connect to the first mail server before the second. Since most spammers will attempt direct connections to each mail server, they will be blocked.
GNU Mailutils is a rich and powerful protocol-independent mail framework. It contains a series of useful mail libraries, clients, and servers. These are the primary mail utilities for the GNU system. The central library is capable of handling electronic mail in various mailbox formats and protocols, both local and remote. Specifically, this project contains a POP3 server, an IMAP4 server, and a Sieve mail filter. It also provides a POSIX "mailx" client and a collection of other handy tools.
Mailfilter is a flexible utility to get rid of unwanted spam mails, before having to go through the trouble of downloading them into the local computer. It offers support for one or many POP accounts and is especially useful for dialup connections via modem. You can define your own filters (rules) to determine which e-mails should be delivered and which are considered waste.