3 Awesome Complete Open Source Mail Servers Solutions For Linux and UNIX servers

In the world of mail servers, there are many software and technologies. Here is a list of 3 of them that you must know. I am going to list software that can turn any Linux, FreeBSD, OpenBSD or Unix-powered into mail server out of a box with the following goals:
* The software must be open source
* The software must run on Linux or Unix-like server
* The software must deploy quickly
* The software setup must be easy for new sysadmin
* The software must support multiple users and multiple domain names
* Must promote privacy and decentralization