msmtp is an SMTP client with a sendmail compatible interface. It can be used with Mutt and other MUAs. It forwards messages to an SMTP server which does the delivery. Features include various SMTP AUTH methods, TLS/SSL-encrypted connections (including support for client certificates), support for multiple accounts, DSN, and IPv6 support.
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.
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.
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.