whew this thing is hard to understand and configure

- https://www.postfix.org/documentation.html
- https://blog.konoson.com/set-up-macos-to-use-postfix-for-sending-automated-emails-from-cron
- https://knazarov.com/posts/setting_up_postfix_on_os_x/
- https://altoplace.com/macos/create-a-send-only-smtp-server-on-macos/