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.
sendmail X is a modularized message transfer system consisting of five (or more) persistent processes, four of which are multi-threaded. A central queue manager controls SMTP servers and SMTP clients to receive and send email messages, an address resolver provides lookups in various maps (including DNS) for mail routing, and a main control program starts the others processes and watches over their execution. The queue manager organizes the flow of messages through the system and provides measures to avoid overloading the local or remote systems by implementing a central control instance. sendmail X is simple to configure using a syntax that is easy to understand and use. It is intended to be used as a secure and efficient mail gateway. It does not provide any mail content modification capabilities such as address masquerading.
asceticsmtp.py is a very minimal "ascetic" SMTP relay-only mail transport agent. It's intended to be used in conjunction with the mutt mail client to provide SMTP mail transport services. It accepts a mail stream on standard input with recipients specified on the command-line, and forwards the message to a given mailhub. SMTP authentication and SSL transport layer security is supported for recent enough Python libraries.
Dkfilter is a DomainKeys implementation for the Postfix mail transfer agent. It is implemented as two filters (or SMTP proxies). There is an "outbound" filter for adding DomainKeys signatures to outgoing email and an "inbound" filter for verifying signatures of incoming email. The filters can be plugged into Postfix as either Before-Queue or After-Queue Postfix content filters. For more information about DomainKeys, see http://antispam.yahoo.com/domainkeys.
BackupMX consists of two programs, 'Store' and 'Forward'. Store accepts SMTP connections, takes all mail given to it, and stores it in a file. Forward takes that file and forwards it over SMTP to a fixed destination. This is code for integrating into your own projects rather than using as is. It is suitable for creating a disaster-recovery mail handler, but would also make a good basis for a spam filter, a tarpit, a honeypot, or a greylist filter. It is written in simple C, and has no external dependencies such as libsmtp.
sendPGPMail.cgi is a small CGI script for sending an encrypted PGP email message to a predefined receiver from an HTTPS secured Web page. The typical use of this script is to provide the users of a secure Web site the possibility to send a secure email to a predefined employee of a company. The CGI script gets its input from an HTML form and passes it on to gpg. The message and its attachments are encrypted using the PGP public key of the predefined receiver.