In the event you missed them, I took a couple stabs at some email tutorials. The intent was to provide enough understanding to get you pointed in the right directions rather than get 'er done copy/pasta recipes. Hence, they may or may not be the droids you're looking for:

Read first: https://community.nodebb.org/topic/15072/nodebb-email-options

Read second: https://community.nodebb.org/topic/15077/nodebb-email-postfix-null-client