OpenSMTPD is a Unix system daemon implementing the Simple Mail Transfer Protocol to deliver messages on a local machine or to relay them to other SMTP servers. It has been released on the 17th of March 2013 with version number 5.3 after being in development since late 2008.
The development of OpenSMTPD was motivated by a combination of issues with current SMTP daemons: difficult configuration, complicated and difficult to audit code, and unsuitable licensing. OpenSMTPD was designed to solve these problems and make mail exchanges accessible to a wider user-base. After a period of development, OpenSMTPD first appeared in OpenBSD 4.6. The first release is targeted for OpenBSD 5.3, as it is now considered stable enough for production use.
OpenSMTPD is an attempt by the OpenBSD team to produce an SMTP daemon implementation that is secure, reliable, performant, simple to security audit and trivial to set up and administer. Code is designed to keep the memory, CPU and disk requirements as low as possible but it is admitted that large mail systems are not intended to be run on low-end machines, and trade-offs are made where higher memory usage would bring indisputable benefits.
As such, the design goals for OpenSMTPD are: security, ease of use, and performance. Security in OpenSMTPD is achieved by robust validity check in the network input path, use of bounded buffer operations via strlcpy, and privilege separation to mitigate the effects of possible security bugs exploiting the daemon through privilege escalation. In order to simplify the use of SMTP, OpenSMTPD implements a smaller set of functionalities than those available in other SMTP daemons, the objective is to provide enough features to satisfy typical usage at the risk of unsuitability for esoteric or niche requirements.