OpenSMTPD is described as 'FREE implementation of the server-side SMTP protocol as defined by RFC 5321, with some additional standard extensions. It allows ordinary machines to exchange e-mails with other systems speaking the SMTP protocol' and is an app in the office & productivity category. There are more than 10 alternatives to OpenSMTPD for a variety of platforms, including Linux, Mac, Windows, BSD and Self-Hosted apps. The best OpenSMTPD alternative is Postfix, which is both free and Open Source. Other great apps like OpenSMTPD are Haraka, Exim, Sendmail and Courier-MTA.
A lightweight SMTP server written in Go, made for receiving large volumes of mail. To be used as a package in your Go project, or as a stand-alone daemon by running the "guerrillad" binary.
Go-Guerrilla is the most popular Web-based alternative to OpenSMTPD.
