Citadel is not very configurable via its configuration file or administrative web interface. If you hate applications with complicated configuration pages, you will love Citadel. If you love applications with complicated configuration pages you will hate Citadel.
Zenen,
I have no experience with Mailinabox, but I am curious about it. Perhaps I will try it one day. As the article points out, I do not really expect to be able to send email to Gmail or the other big email providers. They block essentially all email servers on residential Internet connections. If you want your email to get through to them, some people say to host your email on a server of a commercial hosting company. Sorry I can't be of more help.
A Brief Review of Citadel Email Server Software Running on a Raspberry Pi 3
Zenen wrote
Any opinions on Citadel vs Mailinabox? I'm using the latter but having a lot of trouble sending mail to anyone on Gmail or similar servers.