Are there (any providers of mentoring/training)/resources related to email server configuration?

I was wondering if there are any providers of FreeBSD mentoring/training (or available resources) particularly related to email server setup/administration?

The reason I am looking for mentoring/training is that I want to gain the ability to be able to setup Postfix (for sending and receiving email) on a single FreeBSD server for multiple websites (a couple of websites). I am interested in learning how to do this as I already feel confident (I have servers running already) in setting up web servers (with HTTPS, firewall, etc.) without email services, and being able to send/receive emails seems like the logical next step in the progression of my learning.

Any resources/help/information/advice would be appreciated? I am also willing to compromise and use Ubuntu 16.04 if needed in case resources aren't available for FreeBSD.
Postfix setup for normal use is pretty straight forward since the defaults mostly work fine. Make sure it does not have open access, so search on that to make sure. This site is good:

I run a server for myself and solve security problems by using a tunnel. I'm not sure that will work for multiple users though.
Nothing makes you learn faster than setting up a server and have it compromised and abused within a day :D

After you set up your server you can run some good tests here:

That will at least make sure you're not an open relay. Still not entirely impossible but it's a good start.