A modern, high performance, flexible SMTP server written in async nodejs with a plugin architecture
Plugins provide features like TLS cert file security, dkim, regex selective incoming/outgoing blacklist / whitelist for email addresses and/or domains
If you have experience using postfix this will be a refreshing change
its opensource https://github.com/haraka/Haraka
https://haraka.github.io/manual/tutorials/SettingUpOutbound.html