A friend of mine, Raihan-worked as part of my engineer team @ Excellent-learn Haraka since December as an alternative for simple SMTP engine for our tailor-made SMTP Relay. We would like to build a fast, simple, light, highly extensible SMTP server as a replacement for our current postfix or Zimbra.

What is Haraka?
Haraka is an open source SMTP server written in Node.js which provides extremely high performance coupled with a flexible plugin system allowing Javascript programmers full access to change the behaviour of the server.
As we are currently using Zimbra LDAP as central authentication, Raihan trying to create Haraka as front-end relay with LDAP authentication against Zimbra. Although Haraka comes with docs, wiki and tutorial out there, Raihan took almost 1 month to be able sending email with TLS auth and make sure everything is configured properly.
You can follow his issue thread and how he finally resolved the problem, here : https://github.com/haraka/Haraka/issues/2760