Open sourced A13 Kicad design mail server
https://www.own-mailbox.com/libre.html
Found them on kickstarter: https://www.kickstarter.com/projects/1547898916/own-mailbox-the-first-100-co...
Seems these guys have done some real hard work. As well release their design on KiCAD, they also made a distro that runs on A13 which has a modified mail server that accepts mail replies and posts it at secure https links to standard email addresses which don't offer security built in.
The KiCAD designs are missing some libs due to the way the paths have been hard coded into the project files. I'm sure they will get around to repairing it. Its a more powerful design than a raspberry pi, but many times smaller.