Mailu is described as 'simple yet full-featured mail server as a set of Docker images. It is free software (both as in free beer and as in free speech), open to suggestions and external contributions' and is an app in the Online Services category. There are five alternatives to Mailu for Self-Hosted solutions, Linux, BSD and Roundcube. The best alternative is mailcow: dockerized, which is both free and Open Source. Other great apps like Mailu are Mail-in-a-box (Free, Open Source), iRedMail (Freemium, Open Source), Modoboa (Free, Open Source) and WildDuck (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.