Good morning.
I’m sure it is not helpful, but as “just another datapoint”, I have never had any problem setting up SMTP mail on Discourse in either a Docker-based production install or a desktop macOS “no Docker” dev setup; but I have always used a third-party SMTP service.
You guys are getting my attention; and so now I am very interested in this little puzzle and will try to find time in the next few weeks to set up an SMTP relay on a VPS Ubuntu Linux slice and see if I can get Discourse to play ball with an “old fashioned” SMTP relay versus depending on “third party solutions”.
My apology in advance if it takes me a few weeks to build this configuration. I am falling behind on a bunch of boring government paper work because I have been having too much fun with learning Discourse plugin dev and how Rails works with all the moving parts.