WildDuck is described as 'scalable no-SPOF IMAP/POP3 mail server. WildDuck uses a distributed database (sharded + replicated MongoDB) as a backend for storing all data, including emails' and is a Mail Server in the Office & Productivity category. There are six alternatives to WildDuck for Linux, Self-Hosted solutions, Roundcube and BSD. The best alternative is mailcow: dockerized, which is both free and Open Source. Other great apps like WildDuck are Mail-in-a-box (Free, Open Source), iRedMail (Freemium, Open Source), Modoboa (Free, Open Source) and Mailu (Free, Open Source).
A fully fledged, free email server solution, an open source project (GPL v2). Use official binary packages from Linux/BSD distributions, with both i386 and x86_64 support. Easy, fast deployment in LESS THAN 1 MINUTE. View YouTube Video to check how easy it is.