opensmtpd 6.8.0p2 Lightweight SMTP daemon
OpenSMTPD is an implementation of server-side SMTP, with some additional standard extensions. It allows ordinary machines to exchange e-mails with other systems speaking the SMTP protocol, or to deliver them to local users.
In order to simplify the use of SMTP, OpenSMTPD implements a smaller set of functionality than those available in other SMTP daemons. The objective is to provide enough features to satisfy typical usage at the risk of unsuitability to esoteric or niche requirements.