FreeBSD mail server: rspamd & DKIM

Now that we have our basic mail infrastructure working, we will add support for rspamd and enable DKIM signing of our messages. In this series we will set up a fully-featured mail server in a FreeBSD jail using OpenSMTPd, Dovecot and rspamd. In contrast to many other guides, this one is split into multiple posts that can either be read and followed individually, or as a whole. After each post, you end up with a fully working system (that might lack some features ;)).