Thank you for taking the effort to write this guide. I wonder if with slight modification could be adapted to host the outgoing email too.
I see this guide which isn’t very complicated but it seems redundant to have one container running for receiving and another for sending.
No. Sending mail is very complicated. If you want to do that you’ll need to look elsewhere for instructions and support.
Then maybe it’s the other way around. I did the sending setup and now I don’t want to have a different container to do the receiving. probably I should ask the other side 
You can look in the mail-receiver container for the bit that pushes the message to Discourse via the API.