Run your own mail server
If you’re up to the challenge, a personal (or small network) mail server on an encrypted computer is an excellent way to go. It may expose the participants as belonging to a single service, but can offer privacy and reliability far and beyond what you’d expect from a 3rd party. Many guides are available online, ranging from the step-by-step walk-throughs with detailed explanations to the point-and-click install. Its not a task for the novice but it is definitely doable with some attention and persistence.
- Ars Technica guide to running your own mail server
- iredMail point and click server installation
- Caislean is a set of Ansible recipes that you can use to set up and manage in few simple steps servers offering free and open-source tools for secure communications and information exchange