Mail server support in FreedomBox

Description of the project: Add basic mail server support to FreedomBox. One topic is to fetch emails from a 3rd party service provider, and to store them on a FreedomBox. Main focus is to use existing Debian packages and integrate them into the FreedomBox system. The mail server should receive emails via SMTP and allow emails to be accessed via encrypted IMAP (aka imaps). It would be a bonus if it is able to send and receive emails via Tor (aka .onion addresses), to allow two FreedomBox users to exchange emails without leaking metadata. http://people.skolelinux.org/pere/blog/A_Debian_package_for_SMTP_via_Tor__aka_SMTorP__using_exim4.html describe the SMTP via Tor idea. It would be nice if the server include good ways to filter out spam. An idea is to only allow encrypted emails signed by approved keys.