Nullmailer is a very small, open source and totally free command-line software implemented in C++ and designed for GNU/Linux and UNIX-like operating systems, as a MTA (Mail Transport Agent) replacement of your default mail transport agent.
A powerful replacement for popular MTAs
The software can replace popular MTAs (Mail Transfer Agents), including Sendmail, qmail, and others. It has been engineered in such a way that it can act as a relay-only MTA, which means that it can only be used on hosts that communicate to a fixed set of smart relays. In addition, Nullmailer is easily configurable, easily extendable, and very secure.
Getting started with Nullmailer
To install this software on your GNU/Linux operating system, you should first download the latest version from the official website or via Softpedia, save the archive on a location of your choice, unpack it and open your favorite Linux terminal emulator.
In the terminal window, use the ‘cd’ command to navigate to the location of the extracted archive files and use the “./configure && make” command (without quotes) to configure, optimize and compile the program for your distribution and hardware architecture.
During the configuration step, you can enable support for TLS (Transport Layer Security) and SSL (Secure Sockets Layer) by adding the "--enable-tls" option to the ./configure command listed above (e.g. ./configure --enable-tls). Also, please note that you will have to create a ‘nullmail’ user and group on your system.
Under the hood and availability
A quick look under the hood of Nullmailer will show us that the entire project is written in the C++ programming language. The application’s source code is available for download as a tar.gz archive or via its GitHub page. Detailed instructions about how to configure this software can be found in the HOWTO file inside the source tarball.