Yes, I used the documentation you shared and I was able to setup everything.
I now need to use the mail server to send emails from other frappe sites. How is that done?
everything = Client + Server + RabbitMQ + Agent(s)?
If yes. Then, create a user in the server, generate the API Key and Secret, and put them in the Client - Mail Client Settings. Follow the docs.
There’s a more recent edit with proper chapters and much better audio:
Finding out, that Frappe Mail has stalwart (https://stalw.art/) working as a mail agent 
Btw., stalwart offers jmap instead of imap. As this offer a lot of opportunities and performance, it would be cool, if frappe could support it too.