And to some document giving confidence that it wouldn't be dangerously insecure or vulnerable to common threats?

apt-get install postfix openldap dovecot

I really don't know what the deal is with email, but since the mid 90s there's been this weird thing where everyone wants to follow some kind of step by step guide. But it is just simple software like anything else. If you can setup a webserver or a database server or anything else you can setup a mail server.