Postfix - Postfix is a mail transfer agent (MTA) that routes and delivers electronic mail.
CMail - CMail is a free Windows command line mailer with advanced message formatting options.
mailcow - An open source mailserver suite.
mailsend-go - mailsend-go is a command line tool to send mail via SMTP protocol, written with golang.
Mail-in-a-box - Mail-in-a-Box provides webmail and an IMAP/SMTP server for use with mobile devices and desktop mail software and also includes contacts and calendar synchronization.
sendEmail - sendEmail is a lightweight, command line SMTP email client.