Wikipedia
Haraka (software)
Haraka is an open source SMTP server. Its architecture is plugin-oriented and event-driven. The server and its plugins are written in Javascript using the Node.js framework.
Haraka follows a plugin model, implementing a very lightweight SMTP core and providing developers hooks into the mail flow. Almost all of Haraka's functionality is built as plugins.
Through its use of the V8 JavaScript engine and asynchronous I/O, the author claims the server is capable of handling thousands of messages per second.
Bounce.io announced that they regularly handle 2 to 5 thousand simultaneous SMTP connections with Haraka.