Magma is an email server, supporting many of the common email protocols, designed with security in mind. It was designed and used by the guys at Lavabit (of Edward Snowden fame).

It combines outgoing (SMTP) and incoming (POP, *MAP) functionality, so it would be like using Dovecot and Postfix (rival mail server products) together.

It requires that you have a relational database (the documents describe MySql) and a key-value store (the documents describe Memcache) to support it and the documentation suggests it's designed for Linux (specifically with instructions for CentOS, which is a Red Hat distro). A mail server is generally installed on an "always-on" computer connected to the internet-connected network.

Note that even though it's "designed with security in mind", I don't see any mention of a security audit on either the linked page or the GitHub repo.