WildDuck Mail Server - Self hosted modern mail server `

WildDuck is a modern mail server software for IMAP and POP3. Modern being scalable, unicode-first and API-controlled. You can bundle WildDuck with Haraka and ZoneMTA to create a complete mail server. It uses a distributed database (sharded + replicated MongoDB) as a backend for storing all data, including emails. It tries to follow Gmail in product design. If there's a decision to be made then usually the answer is to do whatever Gmail has done.