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.
fdm (fetch and deliver mail) is a simple, lightweight replacement for mail fetching, filtering, and delivery programs such as fetchmail and procmail. It can fetch using POP3, POP3S, IMAP, IMAPS, or stdin, and deliver to a pipe, file, maildir, mbox, or SMTP server, based on a set of regexps. It can be used for both single user and multiuser setups, and is designed with privilege separation when running as root.
milter-spamassassin is a lightweight, reliable, and fast Sendmail plug-in for the SpamAssassin mail filter. It uses advanced statistical and heuristic tests to identify SPAM built into spamd (the SpamAssassin daemon program). This filter has a hosts/networks whitelist, scans messages less than a defined size, rejects SPAM messages at the DATA stage of SMTP session, copies SPAM messages into the special "garbage" mailbox, performs Subject tagging if SPAM is detected, and adds an information header to scanned messages.
milter-clamav is a lightweight, reliable, and fast Sendmail plug-in for the ClamAV antivirus scanner. It uses a mail scanning engine built into clamd (the ClamAV daemon program). This filter scans messages less than a defined size, rejects infected messages at the DATA stage of SMTP session, and adds an information header to scanned messages.
milter-spamblocker is a lightweight, reliable, and very fast Sendmail plug-in designed to reject spam and Internet worms. It blocks up to 99% of spam and up to 99.9% of Internet worms. You will not need to pay for the garbage traffic now. It has a flexible configuration, real-time sender and recipient email address verification technology with autowhitelist, smart anti zombie technology, safe anti backscattering technology, smart GreyListing technology with autowhitelist, and a low level of false positives.
qSheff is a wrapper for the qmail queue that scans email for viruses and spam. After checking the message, it will wake the qmail queue. It is a replacement program for qscanq and RAV antivirus. It is simple and more powerful. It features the ability to quarantine, very nice logging, subject filtering, a blackhole, easy installation, and white/black lists.