There are a variety of open source simple mail transfer protocol (SMTP) servers available as of 2011. One major consideration when choosing the best from the bunch is the complexity of the setup and configuration of the server itself, because the process can be incredibly technical or, alternately, might be performed completely through a graphical user interface (GUI). Next, depending on the exact use for the open source SMTP server, configurable protection from bulk or spam emails can be vital to preventing the server from being flooded on a daily basis.
If an open source SMTP server is going to be integrated into a larger suite or application, then it might be important to use a server that allows plug-ins or that is configurable so input and output can easily be piped through other applications. As open source software, the state of development of the server, as well as the activity of the developers, also might help in determining the long-term viability of the software.
One of the most important, and potentially frustrating, aspects of using an open source SMTP server is the initial setup, customization and configuration of the server. Some of the most popular open source servers rely completely on configuration files that must be modified in a text editor to change or view options. Other programs give the user control through a comprehensive GUI so options can be set or changed with buttons, dials or a list of valid choices. While there might not be a large difference in the power of the underlying SMTP server, the method the server uses for configuration should be looked at carefully.
If the open source SMTP server is going to be used through the Internet, then customizable filtering options are important. These can include features such as domain name filtering, blacklisting and authentication to prevent random users from logging in and sending mail through the server. Filters that accept regular expressions or other patterns are particularly effective, although they are not a common feature in SMTP servers in 2011.
An open source SMTP server can be used as a standalone server to process mail for a network, or it can be a single part of a larger application or server setup. Not all SMTP servers can be easily integrated with other applications, and that ability is important to look for if it is required. Certain open source servers have support for plug-ins or have source code available that is easily compiled to meet a specific demand. It also might be important to look at what external programs the SMTP server requires to run, because several servers are built using other open source server utilities that will need to be installed and configured for the SMTP server to operate correctly.
For long-term use, it can be important to determine how active the developers of the open source SMTP server are. As technology and protocols evolve, a stagnant open source project might quickly become obsolete. At the same time, the server should be in a stage where it is stable enough so patches and fixes are not being distributed on a daily basis.