femail provides a sendmail-compatible commandline interface and forwards mail via SMTP. Unlike many other implementations, it is fully RFC 2821 and 2822 compatible. It embeds extra environment information from Apache in the header, if present, to help with tracking down abuse. It is written with security as a main design goal.
getmail is intended as a simple, secure, and reliable replacement for fetchmail. It retrieves email (either all messages, or only unread messages) from one or more POP3, SPDS, or IMAP4 servers (with or without SSL) for one or more email accounts, and reliably delivers into qmail-style Maildirs, mboxrd files, or through external MDAs (command deliveries) specified on a per-account basis. getmail also has excellent support for domain (multidrop) mailboxes, including delivering messages to different users or destinations based on the envelope recipient address.
hermes is a generic, transparent and multi-platform anti-spam SMTP proxy that uses a combination of techniques (like greylisting, throttling, DNS white/blacklisting, SPF, etc.) to stop spam from reaching your mailbox. Many mailboxes see a 98% spam reduction with no false positives, as hermes is not filtering email based on content, but on strict adherence to the SMTP standards. It's also compatible with most SMTP extensions like STARTTLS (for SSL security) and SMTP-AUTH (for user authentication).
The inline_smtp Perl script was created to allow filtering of incoming mail on a separate server that houses the mailspools without running a full-blown MTA. While it's possible to configure another MTA, e.g. postfix, to relay all mail to a smart host and do filtering through postfix, the queue can get behind easily, and on a busy host thousands of messages can get queued in a few hours causing delays in delivery from a few minutes to several hours. By filtering while redirecting, mail arrives without any delays, and can be filtered by a seperate host adding headers or content reducing the load on the final mail host.
j-XChange is a pure Java implementation of the entire Collaboration Data Objects (CDO 1.21) library for accessing Microsoft Exchange Server in a platform independent manner. This library will allow interoperability with MS Exchange from all non-Windows platforms that support Java. This library is powered by j-Interop.
libSmtpThread is a multi-threaded C++ library for sending mail via SMTP. The library provides a connection pool that initializes a set of threaded SMTP connections. Emails can be pushed into the pool, which accepts all mail and delivers them to a free connection. The delivery to the SMTP server is processed in a separate thread, so you don't have to wait for mail to be accepted. It has been successfully tested with Postfix.
libspawner implements the MTA side of Sendmail's Milter (mail filter) protocol that is used by the MTA to delegate policy decisions to external daemons. The main purpose of this library is to enable other MTAs, or any mail-handling software, to talk to the existing mail filters that have been developed for Sendmail.