bot-ish pluggable incoming mail processing framework

Schleuder is a group's email-gateway: subscribers can exchange encrypted emails among themselves, receive emails from non-subscribers and send emails to non-subscribers via the list. (Please note: For some platforms there's a better way of installing Schleuder than `gem install`. See <https://schleuder.org/schleuder/docs/server-admins.html#installation> for details.)